oh.my.god.
I can't believe you are even considering just doing one. Unless she is the non mushy type of girl, and she says V Day doesn't matter. But even if she does, get her a little card or something.
A cool idea would be to take her to a 3 min photo booth, get photos done together and then frame and give the set to her.
BUT FOR GODSAKE do not fail to observe both events. Speaking as someone who's birthday can be overshadowed by Christmas, it's not cool to have to sacrifice something everyone else gets just b/c of dates.
/end rant transmission/