I installed an RF660 VPN (note this was originally a 650 that had been upgraded) last week. After 1 week of operation it is reporting disk full errors on the log partition.
Make sure you don't have VPN debugging turned on for both PPTP and IPSec. This can cause the disk to get filled up very quickly. VPN debugging should be DISABLED at all times unless you are troubleshooting a problem. Disk usage will be cleaned automatically when it reaches 80%, so you do not need to do anything.