According to Article 31 of the Civil Code regulations:

Article 31. Right to nationality

1. Each individual has the right to nationality.

2. The identification, change, acquirement, renouncement, or assume of Vietnamese nationality shall be stipulated in the Law on Vietnamese nationality.

3. Rights of each non-nationality resident within Vietnam’s territory shall be guaranteed as prescribed by law.

Besides that according to Article 37 of the Law on Vietnamese Nationality regulations for adopted minors are as follows:

Article 37. Nationality of adopted minor children

1. A child who is a Vietnamese citizen and adopted by a foreigner will retain his/her Vietnamese nationality.

2. A child who is a foreign national and adopted by a Vietnamese citizen will acquire Vietnamese nationality from the date a competent Vietnamese agency approves the adoption.

3. A child who is a foreign national and adopted by parents one of whom is a Vietnamese citizen and the other is a foreign national may be permitted for naturalization in Vietnam according to the application for Vietnamese nationality filed by his/ her adoptive parents and is exempt from conditions prescribed in Clause 1, Article 19 of this Law.

4. Change of the nationality of adopted children aged between full 15 and under 18 years is subject to these persons’ consent.

3. Foreign children adopted by parents, one of whom is a Vietnamese citizen, and the other a foreigner, will be granted Vietnamese citizenship according to the adoptive parents' application for Vietnamese citizenship. and are exempt from the conditions specified in Clause 1, Article 19 of this Law.

4. Changing the nationality of an adopted child from 15 years old to under 18 years old must be approved in writing by that person.

So, if you are a foreigner, the adopted child, the child with Thai nationality, will have Vietnamese nationality from the date the adoption is recognized by the Ministry of Justice of Vietnam.

There are no regulations requiring foreign children adopted by Vietnamese citizens to renounce their foreign citizenship. So in this case the child will have both Vietnamese and Thai nationality.

In case the adopted person is from 15 years old to under 18 years old, that person's written consent must be obtained.