According to point b.2 Clause 1 Article 1 of Circular No. 111/2013/TT-BTC , in case a worker has made residence registration in Vietnam but he/she is seconded to a foreign country to work and is present in Vietnam for fewer than 183 days in the tax year and fails to prove his/her residence in any country, that person is considered a resident in Vietnam. The person has to declare and pay PIT on the whole incomes earned in Vietnam and outside Vietnam according to the progressive tax table.
In contrast, if the person can prove that he/she is a resident in another country, he/she shall be considered a non- resident in Vietnam. This person shall pay tax on the incomes earned in Vietnam according to the tax rate of 20%.
The residency in another country shall be proved by the Certificate of residence or the passport.