I like them both.
Vivienne (sp this way only) is my favorite girl's name. Viv or Vivvie are two nn's I'd consider but probably not use. Well, I'd use Viv just cuz one syllable's convenient when yelling...
I like
Harper b/c of
Harper Lee, the author of To Kill a Mockingbird. It doesn't have any cute nn's that I know of but I don't think it needs.