Aside from what Runestar pointed out, I'd just like to know...how? Kensai lets you add on to an existing enhancement, but you can't use your class ability to increase the weapon's enhancement beyond your class level regardless of if it was already magical or not. So it's not like you're "stacking" the VoP increases with Kensai, and if the goal is to save xp by waiting for a good base enhancement on the weapon from VoP, well...you'll be waiting a long time. And really, at that point, it's not much different than an normal character entering Kensai.
VoP guy: I have a +2 weapon from my vow, so from Kensai 3+ on, I'll be increasing it with xp.
Regular guy: Uh...ditto, except I just payed to make mine +2 before reaching Kensai. *shrug*
In fact...by a strict reading of the rules, VoP may actually be horrible for Kensai. VoP says you treat any weapon you wield as +x. Theoretically, that means you can't even stack it like you could with a normal magic weapon with Kensai. Ie, if you're Kensai 3, you would have to pay for all the xp to get to +3, and then you'd basically have a weapon that's at all times either +3 from Kensai or +2 from vow, but the benefits overlap. And then also...doesn't the weapon chosen have to be masterwork if not unarmed strike? Any non-monk VoP character would be up the creek there...