Diminutives, especially for female names, are to be careful with because some can take a very derisive nuance. Also, many people don't like diminutives or prefer other ones which aren't as obvious.
For Виктория it's very straightforward - Вики.
Мария can be problematic because the common Марче and Марийка could sound quite sarcastic. I think Мими (alternatively Мимка) is safe to use but still, make sure the person (or parents) are okay with them. You can always ask "Как ти викат (на) галено?".