The steps are as follows.
1. As root, do "pon [your_tunnel_name]" (without the quotes, of course).
2. Do "ifconfig -a"
3. Look at the stanza for ppp0. The second line will contain an entry something like this:
inet addr: 192.168.xxx.xxx ...
4. As root, do "route del default"
5. As root, do "route add default gw 192.168.xxx.xxx" (use the value from the ppp0 stanza).
That's it; you should now be connected to your vpn server.