By default, RAS changes the source IP address of packets it routes, from the IP address of the client sending the packets to its own source IP address.
You must disable this functionality in order for LAN-to-LAN routing to function. You do this on the PPTP server by adding the DisableOtherSrcPackets Registry entry with a value of REG_DWORD 0
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services \RasArp\Parameters
This entry prevents the RAS server from changing the source address of packets it forwards.