To stop OS X 10.4 from creating a default route to your VPN connection, open Internet Connect (in Applications). Choose Options from the Connect menu, then uncheck the "Send all traffic over VPN connection" option. Click OK, and you're done.
To make a custom route to the subnet on the other side of the VPN connection, read the rest of the hint...
As root, create /etc/ppp/ip-up, and put in the following code:
#!/bin/sh
# When the ppp link comes up, this script is called with the following
# parameters
# $1 the interface name used by pppd (e.g. ppp3)
# $2 the tty device name
# $3 the tty device speed
# $4 the local IP address for the interface
# $5 the remote IP address
# $6 the parameter specified by the 'ipparam' option to pppd
DEBUGFILE=/tmp/ip-up-debug.txt
## echo "1:$1 2:$2 3:$3 4:$4 5:$5 6:$6" > $DEBUGFILE
NET=`echo $5 | cut -d. -f1,2,3`
## echo $NET >> $DEBUGFILE
case $NET in 192.168.3)
## echo "CASE1" >> $DEBUGFILE
RESULT=`/sbin/route add -net 192.168.30.0 $5 255.255.255.0`
##echo $RESULT >> $DEBUGFILE
;;
192.168.2)
## echo "CASE2" >> $DEBUGFILE
RESULT=`/sbin/route add -net 192.168.20.0 netmask 255.255.255.0 gw $5`
## echo $RESULT >> $DEBUGFILE
;;
192.168.1)
## echo "CASE3" >> $DEBUGFILE
RESULT=`/sbin/route add -net 192.168.10.0 netmask 255.255.255.0 gw $5`
## echo $RESULT >> $DEBUGFILE
;;
*)
## echo "No match" >> $DEBUGFILE
;;
esac
Notes:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060216061850917