A few things that might help you:
If you are considering doing the nightly backup over the VPN, then for any file which changes, you will be transferring the whole file over the VPN. The speed of the VPN will probably be the limiting factor of your backup speed.
A simple way to gauge the size of modified files might be to use Windows Search just to look for files modified in the last day, then select all and properties.
An alternative method might be to replicate the data at your branch office to the main office. This can be done using Replication Exec. This will only transfer the changed portion of any modified files, and the backup takes place on the replica copy at the main site, which will probably be faster.
You can gauge the amount of change data that will be transferred using Volume Replicator Advisor. Though this is designed as a tool to help you with Volume Replicator, it will let you see how much data changes, and when.
See http://www.veritas.com/van/misc/vvr_advisor.jsp
