You need to get vshare license for each of the domains, if you are buying in bulk PM me, we can arrange special price.

You can host any number of vshare sites on a server.

Code:
would affect my current vshare site to have more domains using ffmpeg (like slow it down or anything).
Hosting more sites on a server, it can slow down, it is natural. Video conversion need lot of CPU, so if visitors upload at same time, it can slow down the server.