Vista Business, Wireless and VPN

I've read of a number of people who are experiancing this problem, but
haven't been able to find a solution. I use the built in Windows VPN
client to connect to the office. Was working fine with Windows XP -
whether I was connected to the internet via wired or wireless.

Since upgrading to Vista Business however, it works fine when I'm
connected via wired (no matter from where) but on ANY wireless
connection (tried both my own, and multiple others) it will not
connect at all. Internet connection is fine - I can browse with
wireless, just not connect with VPN.

I've tried disabling the wired NIC, and it's got nothing to do with
switching from wired to wireless - I've tried a clean boot several
times.

Connection to the server is via PPTP, using MS CHAP v2.

I don't want to have to reinstall XP, but if I can't connect to the
office it makes Vista useless for me. Help!