Even on the grand finale night, are you allowed to vote that night?

Yes. Why would you think different?
They (technically, its just Ryan, but still) give you numbers for the two finalists and then you vote for them. Most of the time they will give you three numbers a person, to keep the lines open. You do 186643657 and then what ever number your vote is.

What do you mean by grand finale? The very last show? Because you can't vote then.

i actually dont know but i think you can but only for only when it comes to grand finals of american idol ae