If I use Open the Vaults and there is a Bestow enchantment creature in my graveyard, can I bring it onto the field as an aura? Or just as a creature?
No, you can't use Bestow when you bring a creature with Bestow onto the battlefield without casting it.
If you bring creatures on to the field without casting it, you can't pay the Bestow cost, and you can't enchant another creature that way.
You may cast this card by paying [cost] rather than its mana cost. If you chose to pay this spell’s bestow cost, it becomes an Aura enchantment and gains enchant creature. These effects last until one of two things happens: this spell has an illegal target as it resolves or the permanent this spell becomes, becomes unattached.