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

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

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

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