Will Open the Vaults let me return Bestow creatures as Auras?
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.

Bestow [cost]
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.