As for vows, I would say that depends on the setting a lot.
If the god requires the vows in return for its blessings, then yes. But it the vows are simply part of the structure of the clergy then no (many different orders may have different manners to honor their god - and the god may or may not care specifically how they do it).
I would say the more ordered and organized the god granting the spells is, the more likely the spells come with the vows. Disorganized, chaotic, or apathetic deities are much less likely to require it. But irregardless of the vows, if the character begins to act against the interests of the deity then the spells are likely to stop coming.
Of course this is all also contingent on the spells coming from the deity rather than just a teaching of spells. Someone already mentioned that in some settings spells are spells and the only difference is what group of spells different organizations teach. But the way I am reading your question I suspect you are going towards the divine powered spell idea.
Regards,