There's only one way to figure that out.... Try it and see...
Haven't heard of anyone using it on a Pi yet but I guess if Mumble is usable ventrillo should perform about the same if it's installable. I can't test it for you right away but I suggest to enable the source repositories if it's not available in the binary repo's. That way you can easily install all/most dependencies using apt-get which will save you a lot of trouble if you want to update it later-on. Both the Debian wiki and the apt-get manpage contain a lot of documentation on how to do that exactly.
PS, If it won't run at all, Most people I know that use ventrillo rent a managed server for like $2 a month from the ventrillo team. Also you can use a free Amazon AWS virtual server (or at least for a year). The only thing you need for that is a creditcard to sign-up. The free bandwidth and storage should be more than enough to keep it online 24/7/365
