Squid Transparent Proxy + PPTP VPN == Fail

[thekod]

I can't seem to make squid work transparently through a PPTP VPN.  I googled and searched the forums, and I didn't find anyone else with the problem.  I've followed the chain of includes and functions that (appear) to write the squid related pf rules through to /etc/inc/filter.inc but have come up dry, and so have decided to give up [for a day or two] and see if anyone knows if this is possible.  Also, if I have to add the pf redir rules for the interfaces manually to make this work, where is the function that does that?