Hi Dom,
Just to clarify, AFAIK, media servers can be glitchy on VPS'. They recommend actual physical servers, AKA "bare metal."
For the exact details of what you need in your own instance, I'd check with the media server support staff/forums, i.e. with people who know more about it than I do.
Web conferencing means streaming a lot of RTMP data at once (real time audio, video, text chat, screen sharing, shared whiteboard, etc.) so the load can be high.