I don't know of any modern feminine fragrances that use lavender that would seem to appeal to your girlfriend, maybe Covet by Sarah Jessica Parker. I *think* Jo Malone has a lavender fragrance as well. I was going to suggest good old Jicky edt, but the civet in it is a turnoff to some. I hope you get some more recommendations. If I think of something else I'll be sure to post.
Check out Lancome's Hypnose. Despite the list of notes to me it smells very much of lilac with some sweet vanilla over a slightly green vetiver base. The vetiver here keeps the sweetness in check and gives it a bit more maturity, although it's still a perfume aimed at the younger woman.
Haven't tried any Hilton's for comparison but based on notes you listed:
+ 1 for Nina by Nina Ricci. It's delicious and the bottle's cute, too.
+ 1 Jicky- the EDT strength
+ 1 Lolita Lempicka- original and the L de LL Coral Flowers version. Even my non-perfumista office mates like it.
HHmmm..Maybe some Chanel Frags might interest her, just in general...though I know that Guerlain's Samsara has a nice vanilla note towards it..Maybe just approach a staff member at various perfumeries and tell them what you're looking for, they're most friendly
I'm just mainly concentratig on the lavender note...what about Tauers' Rêverie au Jardin? It's a very flowery,aldehydic lavender..smooth and femenine enough!
I would also like to suggest Gris Clair...but maybe the lavender in the first stages is too sharp...but it's sooo good!