A lot of real life stuff gets transmuted into material for DnD. Although real life religion != DnD religion, there is a bit of simulationist in most games, and you are definitely going to see things based on real life sources. For instance, Elisha drops flame strikes on some soldiers in II Kings 1:10. (There are lots of cool druid/cleric effects in I and II Kings, btw). While I am doubtful of the historicity of the passage, it is certainly a real life text.
Acts 3:6 has Peter saying to a crippled beggar: "Silver and gold have I none, but such as I have give I thee: In the name of Jesus Christ of Nazareth rise up and walk." And the beggar is healed. Or maybe
healed.
The story goes that Thomas Aquinas surprised Pope Innocent II as he was counting stacks of money. The pope said, “Brother you see that Peter can no longer say ‘Silver and gold have I none.’”
“Quite true,” replied Aquinas, “But Peter can also no longer say to the lame man, ‘Rise up and walk.’”
Sometimes it's St. Dominic being given a tour of the Vatican. In other versions of the story it is St. Francis. The pope is usually unnamed, but I've seen Innocent II and Innocent IV. The details don't really matter; it's a cool story.
And, without gettng sidetracked with real life religion, it illustrates how people might want to associate poverty (lack of silver and gold) with divine gifts. So if for some reason you are modeling miracle workers (especially saints or apostles) in D20, you might link clerical spellcasting with taking (and living) the appropriate vows.
That said, I suspect it might be a good idea not to have the VoP be quite so black and white. To borrow from real life rules you might have a "mitigated vow of poverty". Or a "simple vow" as opposed to a "solemn vow." Maybe you could compare the character's wealth to the wealth by level guidelines, and give him the difference in vow related powers.
For instance, if a 10th level character with the vow has 6th level wealth, he has the VoP benefits of a 4th level character. They would also need permission from their superiors to use particular items, and the use would have to be justified. A holy sword to slay the BBEG, etc.. Not just any bunch of loot. The character would not actually own the item; he couldn't sell it or give it away, and he would have to return it at his superior's request.
I don't know if this would be balanced, or even makes sense (I gave away my copy of BoED) but something like this might work in some cases. But not in others: if you are running a D20 modern game with low magic, you'd probably want to require the full vow, and give nothing for mitigated or simple vows. Lots of people in real life take a vow of poverty (I'm one of them), but it almost never applies as strictly as the BoED describes it (I have access to a computer, for instance, which I am using right now). And almost none of these people have magical powers (at least, I don't!

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