All that is because the Vow of Poverty isn't really a vow at all. It's a) a PR tool to make people who know what Grey Wizards can do not shit themselves, b) an educational and corralling tool to limit excesses, put fear into Journeymen, and simultaneously pressure some into useful and Ulgu-conductive double-think while also keeping control of those who can't do that, and c) a readymade excuse to audit any member at any time and deal with anyone problematic. Ask any Karaz Ankor Dwarf whether they think Mathilde might have sworn the Vow of Poverty and they'll laugh you out of the room. Give them the wording and insist that the Grey Order has all its members swear this and they'll call you a liar and possibly declare a Grudge on you. Actually prove this to them and they'll lower their respect for Mathilde and consequently humanity as a whole by a noticeable amount.
Or maybe not quite. I don't remember the actual exact wording of the Vow (or even whether we ever got that). But I do remember that it's wording is at least supposed to very much not seem to outsiders as if it is leaving loopholes large enough to drive a mountain penthouse through.