You can create a batch file including robocopy command to compare the data and copy new and mortified files only. This case study may help,

VPN Case Study Synchronizing data automatically via VPN  VPN client can ping the VPN server only  VPN Error 733 - the PPP control protocol for this network protocol is ...
     www.chicagotech.net/casestudy/vpncases.htm  

Bob Lin, MS-MVP, MCSE & CNE
Networking, Internet, Routing, VPN Troubleshooting on http://www.ChicagoTech.net 
How to Setup Windows, Network, VPN & Remote Access on http://www.HowToNetworking.com 
 "Have three columns R,S,T" <HavethreecolumnsRST@discussions.microsoft.com> wrote in message news:9E56C31F-997B-40C8-A615-BA49AD19B234@microsoft.com...
 Greetings all,
 My laptop is connected thru the office network, while I am physcially in the
 building. At laptop shutdown, I sync everything and all is well. So, now I am
 working from home and using VPN to access my office system.
 How can I sync while not physically connected to the system?

 As usual, thanks for any advice.
 Doug