I myself did it with the ppp-over-ssh method, just because all necessary programs (pppd and ssh) were present at both ends and so I only had to do some easy configuration.
So, it is the easiest, but not the best solution. Besides, the gateways between the two networks should be unix ones. (For me it was not a problem as I use Linux at home for connection to the office. I also connected two office networks which had FreeBSD gateways).
If you are interested in this solution, you can find some more information at an older thread of mine at this forum.
However, do not use the ppp-over-ssh method if the vlan is "mission critical", as it tends to hang sometimes (I myself use a guard script to restart it if hung).
There are some other free programs that are designed for the purpose and thus they are much more reliable; however, their configuration might be harder. Unfortunately, I cannot recall the names of these programs. Google searching might help.