(Yes, 3E VoP also made you mechanically weaker; but no where NEAR as badly as PF's version does; and at least in 3E's case it was cause they simply did not realize the value of wealth while as with PF they intentionaly AND EXPLICITLY set out to make the option godawfulhorrible to punish people stupid enough to take it!)
I thought the justification the designers gave was "You're giving up all your items, if that didn't make you far weaker then it wouldn't make sense fluff-wise. Only a munchkin would want to gain actual benefits from that."
Also the vows replace Still Mind, meaning that being more monk-ish than normal stops you from entering monk PrCs.