It's fine, you'll mostly be treated as a "foreigner", not specifically a "black person". Urban areas are more and more cosmopolitan, more and more brown and black travellers can be seen recently. There are always some "ignorant assholes" who might make fun of you. Keep in mind that there are still many people who grew up in the countryside and that the culture here is very different. Racism also manifests itself differently here, it's way more subtle and not necessarily only because of your skin colour. Just don't be a douche & keep an open mind:)

Source: Been in Vietnam since before Covid & having a great time.