If you give out wealth at or above the standard wealth guidelines in such a way that the player can convert all the wealth into useful bonuses like the ones the vow provides, the vow will be underpowered. Otherwise, it is potentially overpowered. For example, a Monk/Paladin would be very happy with stat boost items for every stat except Int. With VoP, he's going to be harshly limited on which stats can be boosted, since it doesn't give very many stat boosts. However, if you weren't going to give him those stat boosters or the money to buy them anyways, then it is no longer a weakness.
Basically, you can see the VoP as a preset purchase plan for magic items that costs two feats and causes a bunch of other inconveniences, and in that sense, it isn't so strong compared to standard wealth.