In terms of actual power level, it seems like a lot to most people just because the person who's taken this Vow winds up getting tons of bonus feats (more than even a Fighter, I believe, although I don't have my Book of Exalted Deeds right in front of me), and they get a lot of stat boosts and immunities. It boils down to the fact that they're getting things that none of the other classes get. Combine this with a Monk, and you've got a pretty fearsome character.
The flip side of things is that the character owns nothing. They have the clothes on their back, and that's about it. Not even a backpack. Definitely no magical items.
The party getting upset about the character with this Vow giving up their share of treasure to charity is neither here nor there. A party should have no say in what a character does with their share of treasure once the treasure is portioned out. It'd be like the Rogue telling the Paladin, "Here's your share of the treasure. But you can not purchase a Holy Avenger with it!" I mean, c'mon. once the shares are doled out, those shares are the direct property of the shareholder. The rest of the party has zero input on what that character does with their funds.
While it isn't required by 3rd Edition rules, I know several Dungeon Masters who still enforce the old 1st Edition and 2nd Edition tithing rules for Paladins. They have to give up 10% of their earnings, both monetary and magical. So what does the party have to say about the fact that the Paladin, once he/she has their share of treasure, goes off and immediately gives 10% of it to an organization that the rest of the party may not be interested in subsidizing? It doesn't matter; that share of the treasure is the Paladin's, and the rest of the party can sod off if they don't like what the Paladin does with their own funds. Likewise, a Paladin doesn't expect to tell the Rogue that they can't purchase a magical set of thieves' tools, they can only say, "You'd better not let me see you doing anything illegal with those."
Don't worry about it; if your party is that concerned with what everyone is doing with their own funds, they need to stop being so nosy, and let the character be as long as they're positively contributing to the group's survival and viability. Someone who's taken the "Vow of Poverty" feat definitely does that.