I like
Victoria a lot and
Veronica not at all, which probably doesn't help you. But, because I've got a very good friend whose given name is
Vickie, that's what I'd use.
I think looking at nns is a useful exercise.
Vickie or, local politics permitting, Torie would be fine for
Victoria.
Veronica could give you
Vera,
Ronnie or Nickie. So as long as there wasn't a risk of
Vickie and Nickie, and you are OK with the other possibilities, the names themselves aren't similar except for starting with V, ending with -a and being 4 syllables long.
If you do use them both, then for everyone's convenience you should give them different initials - preferably two mns each, to cut down on confusion even more.
Victoria Caroline SuzanneVeronica Eloise Mary
Victoria Ruth MarinaVeronica Alison Jean