Backing up over VPN is a bad idea.
It is bad because your bandwidth will be your limitation. In my common encounter, 3 Mbps is the fastest that we can get. That is way more slower than writing to a SATA disk, usually at 3Gbps
Your backup will never finish.
What you should do is having a local BackupPC server, that backs up all machine, then run rsync to copy the backup data to remote server. You may not have latest and complete data, because new data keep overwriting the old one, but at least, you get a copy somewhere remote. If your machine did not change data much, your rsync will finish in 2 or 3 days.
This is how I implement a secondary backup for my client.
One machine as BackupPC, then I run rsync to copy /var/lib/backuppc to another NAS. I made a script to check if the rsync is running every hour. If there are no rsync running, start a new session. With nearly 1TB data, usually the rsync finish copying within 24 hours (that is in same network). If on VPN, I think it could take 1+ month (100 Mbps network = 24 hours or 1 day, 3 Mbps VPN = 100/3 = 33 days)
