I prefer
Vivien. I think it's a good name, and it's the right time for it.
I think
Vivian is almost too masculine for a woman (but not really).
Vivian is totally unisex to me.
Vivien looks less stern - the e gives it some feminine vanity and the right amount of quirk. Either way though, it's a rather bold, dominant, and even severe sounding name in real life - at least that is my impression of how it mis-fits the friendly, goofy young
Vivian that I know.