Vai com Deus
This expression is really common. It is used by professors to students when they go home, by parents to their children when they leave the house, or by someone that is just saying good-bye to somebody else.
The literal translation would be to go with God. It means to take care, to have a safe trip, to go well. It may even be used by people who are not religious, as it is a very common expression in Brazil.
Note, however, that it can also be used in a sarcastic way, when someone is not wanted somewhere and then finally goes away.
- Tchau mãe.
Tchau meu filho. Vai com Deus.
- Então você não vai mesmo pra casa dos meus pais? Então eu vou sozinho!
- Isso, vai! Vai com Deus! Já vai tarde!