The basic answer is not with Apple equipment.
You mean VPN not VLAN.. but that is just terminology issue.. the fact is the TC has no VPN.. so unless you are prepared to get a real vpn router, and put the TC into bridge it wouldn't work. TC is really a home device.. it is not designed for the kind of backup functionality you would need to do this.. ie rsync.
Let me also say I would hesitate to do TM backup over internet. It is a noisy protocol with a lot of chatter back and forth so it is slow without ethernet.. just using wireless in home setup TC is slow. Over your upload speed of 1mbit or less it would really take ages. (that is assuming you are without fibre or really fast cable).
Why not look at cloud backup.. mail server doesn't have to be a huge backup. And use a better backup software designed for cloud use.
Or backup to USB drive and swap drives on a weekly basis so you have an offsite, full backup then use cloud for incremental.
While Time Capsule isn't ideal for this application, you may want to consider an offsite backup solution like Zetta.net's DataProtect.
It gives you offsite replication of files (including Macs, Windows, and Linux) from both your offices for an affordable price. Also, since it's a managed service you have access to 24x7 phone support, which is very handy if you ever have to use it.