Vista can't authenticate on VPN connection

Hello,

I've read up quite a bit about VPN problems with Vista, but can't seem to
find a solution to my issues. We have VPN setup to our Cisco PIX 515E
(which doesn't support MS-CHAP V2). Of course, since Microsoft was nice
enough to remove MS-CHAP V1 in Vista, this now prevents any of our users
from upgrading to Vista, and I'm trying to find a workaround.

Right now, I've made changes to our PIX to allow authentication over PAP,
CHAP or MSCHAPV1. The PIX has always required 128bit encryption. (This
according to the MS KB article discussing the death of MSCHAP V1, should
work).

In my VPN connection security , I've tried every combination of PAP, CHAP
and the various data encryption options, but can't get beyond the dreaded
"Error 732: Your computer and the remote computer could not agree on PPP
control protocols". I don't see anything interesting in the PIX logs or in
the Windows Vista client event logs.

User authentication is being done by an IAS server that the PIX connects to
just fine. Clients running XP, 2000 and OS X can all VPN in without any
problems at all.

Has ANYONE gotten Vista <---> PIX VPN working at all with the Vista VPN
client?