I think using
Vivianne is a nice way to honour your MIL. Spelling it
Vivianne should encourage people to pronounce it how you want it pronounced, but it won't be foolproof and some/most will still pronounce it like
Vivienne. Would that bother you to have to keep suggesting to people how to pronounce it?
Vivianne Alice is a lovely combo and it goes well with
Ruby and
Gwendolen - great sibset!