In statements only tai can be correct. Vai can be used only in questions. Have a look at this:
Hi Marcin! :-) Long time, no see...
Hang on, I'm trying to get this right: In a sentence like this, tai means that it's either A or B, but not both. In a question, tai can mean neither A nor B or either A or B. I'm thinking about whether it could even be both A and B... With vai, it's always only either A or B.