Is asking the girl you really like to be your valentine the same thing as askin her to be your girlfriend?
Well sort of the girl will usually expects you to stay with her after not just ditch her after valentines. Just let her know its for the day if you don't want it to go past that
If you make a big deal out of it, yes. If you give her AND other girls valentines- no. Make sure she doesn't think it's more.
why would you only be together for the day?
Think of it as a trial run. If after valentines day you want to be with her, tell her.