Self-assessment in Vietnam: how returns, supplementary filings and your tax account fit together

Vietnam taxes companies on a self-assessment basis. The company calculates its own liability, files the return electronically and pays by the deadline; the tax authority checks afterwards. When the company later finds a mistake, it is expected to correct it itself by filing a supplementary return — and it pays late-payment interest on any tax that turns out to have been paid late. The system rewards companies that find their own errors first.

For a foreign-invested enterprise (FIE) used to a system where the authority issues an assessment, two consequences are easy to underestimate. First, there is no annual "all clear": a return that was accepted by the system has only been received, not reviewed. Second, the company's e-tax account is the authority's view of what you owe and what you have paid, and it is worth reconciling as carefully as a bank account. This guide walks through the cycle and the correction mechanism.

The cycle: file, pay, reconcile

Every recurring tax follows the same loop, set by the Law on Tax Administration (Law 38/2019/QH14):

1. Prepare the return from the accounting records — the VAT return from sales and purchase invoices, the PIT withholding return from payroll, the foreign contractor tax return from payments abroad.
2. File it through the e-tax system, signed with the company's digital signature. The system issues an acknowledgement and then a notice of acceptance or rejection. Keep both.
3. Pay through a bank connected to the tax payment system, quoting the correct tax code, tax type and period.
4. Reconcile the e-tax account against the ledger: every liability declared, every payment credited, no unexplained balance.

The deadlines under the Law on Tax Administration are the 20th of the following month for monthly returns, the last day of the first month of the next quarter for quarterly returns, and the last day of the third month after the year end for the annual CIT finalisation. Whether VAT and PIT are filed monthly or quarterly depends on criteria in the rules, mainly the size of the business; newly established companies should confirm their filing period at registration rather than guess.

Accepted does not mean agreed

The e-tax system checks that a return is complete and correctly formatted. It does not check that the numbers are right. A company can file a VAT return that the system accepts in seconds and still be asked, months later, to explain why its declared output VAT does not match the e-invoices it issued.

The authority's review happens in two ways. A desk review at the tax office compares your returns with other data the authority holds — e-invoices, customs declarations, bank information, returns of your counterparties — and may lead to a request for explanation. An audit at the company's premises goes through books and documents for selected periods. The risk-based selection of taxpayers is governed from 1 July 2026 by Circular 94/2026/TT-BTC on compliance management and risk management; we name it for reference and do not summarise its criteria.

If a request for explanation does arrive, treat it as the start of a conversation rather than an accusation. Read exactly which periods and figures it asks about, reply within the time it gives, attach the reconciliation that explains the difference and, if the difference is a genuine error, file the supplementary return at the same time. A clear written answer that closes the point usually ends the matter at desk level; an unanswered notice tends to become a reason to select the company for an audit.

The practical lesson is to review your own returns with the same data the authority uses, before it does.

Correcting mistakes: the supplementary return

When a company finds an error in a return already filed, the normal tool is a supplementary return (tờ khai bổ sung) for the period in which the error occurred. The company declares the corrected figures and the difference:

• If the correction increases the tax payable, the company pays the additional tax together with late-payment interest from the original due date to the date of payment.
• If the correction reduces the tax payable, the overpayment can generally be offset against other liabilities or refunded under the rules.
• For VAT, some corrections — such as input VAT on an invoice received late — have their own rules on which period to declare in. Check the current guidance before deciding whether to amend the old period or declare in the current one.

Timing matters. Corrections made voluntarily before the authority announces an audit or inspection are treated differently from adjustments made after an audit decision. Once an audit has been announced for a period, the room for self-correction narrows. We do not set out the penalty regime here; Decree 125/2020/NĐ-CP on administrative penalties for tax and invoice violations is the text to read, together with any later amendments.

Late-payment interest, with a worked example

Late-payment interest runs per day on the amount paid late. The rate has long been 0.03% per day; check the rate in force for the period concerned before relying on it.

Hypothetical example. Suppose a company that files VAT quarterly discovers in August 2026 that a sales invoice of VND 200 million, with VND 20 million of output VAT, was left out of its Q1 2026 return. The Q1 return and payment were due on 30 April 2026. The company files a supplementary return and pays the VND 20 million on 20 August 2026, 112 days late. At 0.03% per day, the interest would be:

VND 20,000,000 × 0.03% × 112 = VND 672,000.

The amount is small because the company found the error quickly. The same omission found by the authority two years later carries interest for roughly six times as many days, plus whatever consequences follow from an audit finding. That arithmetic is the whole argument for a monthly reconciliation.

Your e-tax account is a ledger: treat it like one

The e-tax account shows, by tax type and period, what the authority believes you declared, what you paid and what remains. Differences between that picture and your books come from ordinary causes:

• A payment made with the wrong tax type, period or reference, so it sits unallocated or against the wrong liability.
• A payment by a branch credited to the head office, or the reverse.
• Late-payment interest calculated by the system that the company has not booked.
• A return rejected for a technical reason that the company believes was filed.
• Balances left over after a change of managing tax office.

Each of these is easy to fix in the month it happens and hard to fix years later, when the person who made the payment has left and the bank records are archived. A monthly check — ten minutes comparing the account balance by tax type with the general ledger — is the cheapest control an FIE can run. Where the system shows an amount as outstanding that you have paid, request a correction of the payment record with the payment proof attached.

Records to keep for each filing

For each return, keep in one place: the filed return, the system acknowledgement and acceptance notice, the working papers that support the figures, the payment proof, and any correspondence. Accounting documents used directly for bookkeeping and preparing financial statements must be kept for at least 10 years under the Law on Accounting. For e-invoices, keep the original electronic files rather than PDF copies.

When staff change — and in FIEs finance staff change often — this file is what allows the successor to answer a query about a period they never worked on.

Where to check your own position

• Your e-tax account — returns filed and their status, payments credited, balances by tax type and period. The mobile tax app shows much of the same in read-only form.
• The e-invoice portal — invoices issued and received, to compare with your VAT returns before the authority does.
• The taxpayer information lookup — your registration status and managing office.
• Your managing tax office — for payment corrections and questions the systems cannot answer.

Our sister site CongThue.com (in Vietnamese) walks through the e-filing steps screen by screen.