Vista: PPTP VPN over WLAN

I have a strange problem with Windows Vista (Business and Ultimate Editions).
We have an ISA 2006 Server with PPTP dialin. The ISA server is protected by
an openbsd front firewall which redirects all pptp related traffic to the ISA
box.

The PPTP dialin works fine with XP clients. With Vista I first couldn't
connect via PPTP at all. After updating the front firewall to the latest
version of openbsd, I achieved to connect, but only via ethernet. If the
Vista client is connected via WLAN to the internet no connection can be
established. I am quite sure that the problem is still related also to the
openbsd box. I suppose that the GRE traffic is not correctly forwarded if
Vista communicates via WLAN. Perhaps it is something with packed
fragementation.

My question is: Does anybody have an idea what kind of difference might be
in communication via ethernet an WLAN. Perhaps it has something to do with
Path MTU discovery.

Any suggestions, ideas and comments?

Thank you,

P.