Vivienne Harper is my first pick, second is Vivienne Winter.
I wouldn't worry about Vivienne being too mature for a little girl; she will be a little girl for 10ish years, but you are choosing the name that she will have for her whole life and she will be an adult for many times over how long she will be a child. You can always use nicknames/diminutives while she is little if you feel it is too adult (although I think it is a sweet name for a child anyway) and then she has a beautiful elegant name for when she matures.
I agree with lrmum, you're naming a woman! she will only be a girl for a (relatively) short while in comparison to her adult life. That being said, I still love Quinn Juniper. Vivienne Winter is also a really great choice, imo.
You can always wait until she's here, and see which suits her best!
good luck, hope you find your solution :)
Oh wow Vivienne Winter is very cool! Still loving Quinn Juniper too. Vivienne Hadley and Vivienne Harper are another two gorgeous combos!
Maybe get down to a top combo for Vivienne and a top combo for Quinn, and then see how you feel when you meet her? I prefer Quinn overall, but prefer Vivienne with your boys names. Vivienne is a little more grown up but there's absolutely nothing wrong with that as she will be a grown up eventually, and for most of her life. Anyway that's where nicknames come in handy! Vivi would be adorable on a little girl until she's old enough to really own Vivienne!
I'm not sure about suggestions of other first names but I'll try! You seem to like "n" ending names so I'll start with those:
You also seem to like unisex or surname names with a feminine tilt, or strong girl names, so how about:
Juniper (as a first?)
Hadley (as a first?)
Hope there's something in there, or that you start feeling more sure about Quinn and/or Vivienne! Best of luck and please let us know what you decide :)
Hadley stood out as being just a little different. I sounds pretty with Vivenne too!