i'm using vpn intensely， but when i'm in office, i still need the connection to my local network, so here is the solution

ppp0

#!/bin/sh
# put this in /etc/ppp/ppp0
# mac user only

gw=`netstat -nr|grep UGScI|awk '{print $2}'`

# replace 192.168 with your local network
/sbin/route add -net 192.168 -gateway $gw