If it's in the downtube how do you access it for setup and maintenance?
It's in the seat tube.
There's not much about fitting on the website, but you need a custom bottom bracket, and I don't see how you can do a lot more than drop the motor into the tube, which then meshes with the gear on the bottom bracket.
The motor shouldn't require a lot of maintenance, but presumably it would drop out if you up ended the bike - having first removed the saddle post.
The motor must also have wires to it for electric power, so you could possibly yank it out using those, although it's not clear to me how the wires are routed.
I suspect you need a saddle post which is open at the top, which would rule out some types of saddle mounts.
Incidentally, the site says 'carbon - on request', so it looks like there may be a way to fit a Vivax to a carbon bike.
http://www.vivax-assist.com/en/product/technology/retrofitting.html