As long as you flew into Hanoi where you applied, I cant see why you cant travel within the country. There's no exit stamp in your passport. If they were to call you in for an interview at the embassy, you should be able to get back quickly. Have a good time in HCMC!
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.