Damn I didn't mean to sound like a salesman, I've used Veritas Arcserv and Legatto in the past, I've just been impressed by Acronis so far. Now I'm pretty certain there is a continual backup option (I read something about it but I've not tried it). I've no idea how much Double Take is, Acronis is about 999 of your earth dollars (I'm a Brit) and I'm pretty certain there is a volume scheme.
As to if Acronis will backup across the VPN sure it will, assuming no network problems :-). There is a free trial of Acronis
Backup software for data backup and disaster recovery in Windows and Linux - Acronis better still you can ask someone more detailed questions there as well, they have a IM Chat thingy..
Isn't Double take something you use in conjuntion with other backup software, not used it so I could be talking Bollo.