I have a V2 850G with DD-WRT V24Sp1 Mini on it The differences between the versions of the Routers is how much ram it has on it, V1= 4mb, V2/V3 =16mb
As for the VPN aspect, the 850 should be able to run it as the it has enough ram to support it, but Wait until SP2 of DD-WRT is released in June, SP1's bugs are annoying to say the least
