[name_f]Juliet[/name_f]/[name_f]Juliette[/name_f] would be my choice, I absolutely love that with [name_f]Vivienne[/name_f]! All of them are lovely names, though 
All gorgeous names! And I love your first daughter’s name! 
I’m loving [name_f]Juliet[/name_f] and [name_u]Eloise[/name_u] with big sister [name_f]Vivienne[/name_f]. I love [name_f]Margot[/name_f] too, and while it’s posh like [name_f]Vivienne[/name_f], it doesn’t share the same level of sophistication and elegance, in my opinion. [name_f]Marguerite[/name_f] nn [name_f]Margot[/name_f] would complement [name_f]Vivienne[/name_f] more than just [name_f]Margot[/name_f] on her own. Again, in my opinion.
As for [name_f]Juliet[/name_f] and [name_u]Eloise[/name_u], it’s a toss up for me. Both work well with [name_f]Vivienne[/name_f].
[name_f]Juliet[/name_f] is a beautiful baby sister name that compliments big sister Vivenne perfectly.