What addresses are being used for the PPTP connection? Does the server box have a route to send packets back to you over the VPN? Can you ping the server box from your end of the PPTP connection?
Suppose your VPN uses 10.1.1.1 and 10.1.1.2 at each end of the tunnel. Then if you are connecting from 10.1.1.1, the server needs to know how to route packets back to that address. It would probably need to have a static route added for that purpose.
If you control the PPTPD server, you might try adding a masquerading rule to rewrite the packets' source address with its own. Something like:
assuming eth0 is connected to the network where the server resides, and your PPTPD connection uses ppp0 for its interface. This tells the kernel to take traffic designated as going from the PPTP connection to the wired network and replace its source address with the machine's own address on the wired network. It will then pass the replies it receives back to you.
/sbin/iptables -t nat -A POSTROUTING -i ppp0 -o eth0 -j SNAT --to-source ip.addr.on.eth0
Edit: Also what about on your end? When the PPTP connection is up, is there a route added to your machine that either makes the PPTP connection the system's default route, or adds a specific static route from your machine to the server's network? If the server resides at, say, 192.168.1.10, you'll need to have a route on your computer that tells it to send traffic for the 192.168.1.0/24 network via the PPTP connection. Something like
assuming 10.1.1.2 is the remote end of your PPTP connection.
/sbin/ip route add 192.168.1.0/24 via 10.1.1.2