Vista SP1/SP2 VPN Fails to Attempt to Connect After Switching Networks

I've scoured the Internets for hours for a similar problem, but every other Vista VPN problem is slightly different from mine.

I'm a consultant and work on and off client sites.  One particular client's network is severely locked down (can't VPN out, heavy Internet filtering), but I do have access to a "dirty" wireless network in the same building (it's actually a different ISP entirely).  I switch between their wired network and wireless network (wired so I can RDP and use VSS on their network and wireless so I can VPN to my company's office for our VSS and Exchange for e-mail).  I am not allowed to be connected to each simultaneously.

Here's the issue: Every time I switch from the wired connection to the "dirty" wireless connection, I lose the ability to connect to my office's VPN.  It fails immediately with an 800 error.  I'm convinced that it doesn't even send a single packet out before it fails. To make it work, I must reboot while disconnected from the wired network (ethernet cable unplugged), then connect to the wireless network after the machine boots back up, and then I'm able to connect to the VPN, but only on a fresh reboot like this.

EDIT: If I make a successful VPN connection while on the wireless network, and that wireless network flakes out, once it reconnects, the VPN will consistently fail to reconnect until I reboot.

Logoffs and relogging on doesn't change anything.  Disabling and re-enabling or resetting the ethernet adapters doesn't fix anything.  Renewing IPs, flushing DNS, setting static IPs.... none of it works.  ONLY a reboot allows me in, and as soon as I connect to the wired network it all dies.  I CAN reconnect to the wireless network without a reboot, just no VPN.  This happened after SP1 as far as I can remember.

More interesting yet is that in Vista's Network and Sharing Center, of the three options ("All," "Dialup and VPN," and "Wireless") once I switch from the wired to the wireless network (no reboot), "Dialup and VPN" option disappears completely from that list.  I have a shortcut to the VPN connection that I try to launch.

I have NO virus software even installed, and I have NO firewalls enabled whatsoever.  I've looked through the diagnostic logs and there's nothing interesting in there -- no other errors except the 800 one the VPN dialog returns.

So, does anyone have any ideas?  Anything I could try -- registry hacks, third-party VPN clients, anything?
Cory_Larson04Asked:
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gamma1983Commented:
My remote sites I learned to watch out for spi firewall on linksys routers and to tweak the MTU when there is too much overhead.
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Cory_Larson04Author Commented:
I have resolved my issue.

Vista SP2 + PPTP & L2TP driver upgrades + NIC driver upgrades (Realtek device) solved the issue with switching between networks, but it did not solve my connectivity issues with VPN.

After the previously mentioned upgrades, I noticed that I could successfully connect to the VPN from my older Linksys router at home, but not from the newer Linksys by Cisco routers that exist on my client site.

A firmware upgrade and warm-reset of the router at the client site fixed the rest of my issues.  In case you find yourself with the same trouble, here's the equipment I am working flawlessly with now:

- Gateway M-1625 with Vista SP2 and latest drivers for every last device, virtual or otherwise
- Linksys by Cisco WRT110 v1.0 router, now with 1.0.05 firmware
- VPN passthrough at work is integrated into a Netgear router, just FYI
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