"Excuse me, do you speak either French or English?"
Translation:Anteeksi, puhutko sinä ranskaa tai englantia?
Puhutko ranskaa tai englantia? = Do you speak either French or English?
Puhutko ranskaa VAI englantia? = Which one of these do you speak: French or English?
Vai is for when it's a choice between two options, like, "Do you want chocolate or vanilla?"
Tai is for when it's an and/or situation with a yes/no answer, like, "Do you have any milk or cream?"
Edited to add: This is just when it's used in questions. Vai is used only in questions, but tai is used in questions AND in statements, like "That bottle has either milk or cream in it."