Time to roll a new character
Seriously, though... if your GM thinks that that VoP is powerful enough to warrant a +4 LA (AKA "kick me" sign), you're not going to be able to convince him. You can take the most well-reasoned arguments and they're going to bounce. BTW, there have been threads both here and at the Wizards boards that have such arguments. But hey.
Now, the VoP benefits are huge (esp. for a monk or a shifter druid), but the drawbacks are also huge. I think the GM is seeing the benefits without the drawbacks. First, there's the limitations on items. Not using stat boosters... eh. Not being able to use one-shot items is enormous. Can't feed a potion to your buddy. DR is going to _suck_ unless it's magic or adamantine. Now, a monk looses less than most because your class abilities balance it out to a degree... but the second time in a day you're stuck in a Forcecage is going to make you wish for a Helm of Teleportation.
Then there's the roleplaying. Exalted is all about the ends _never_ justifying the means. Kill a child to save a city? Sorry, won't do it. A world? Sorry. The universe? Nope. If the GM can get you into situations where the restrictions matter, then you're going to see a downside of the VoP (or any Vow for that matter). If it never comes up, then... well, it never came up.
I played a VoP wizard for a while (let's ignore the question of "spellbook", it was finessed by the GM). The adventures ended up never really making a big deal of it... and it ended up being overpowered and boring. Now, a wizard is arguably one of the weakest VoP classes; if I'd been playing a monk or druid, it would have been _really_ overpowered and boring. The roleplaying restrictions are integral to the balance of the ability.
I'd suggest a compromise. You want the roleplaying aspects of the VoP, and the GM doesn't like the mechanics. I've always been a big fan of handwaving in these situations. Why not allow the monk to sacrifice/give away/whatever his share of the treasure, and basically have "virtual items" that he "buys" with that donated gold. That way your treasure will match everyone else's. They're the result of favor by a god or something. You'll take a big hit to AC (one of the reasons that VoP is so good for a monk or shifter...), but you'll also get the ability to customize. It might work out okay.