Some of the community guys tried to get a mumble server up and running in Warband but it never took off.

So then they tried a TS3 server which also failed.

Majority of gamers like vent. That coupled with most older groups having static addresses makes it easier for people to get in touch with each other.

I just read your reasons for liking mumble over vent.
Vent does everything in your list except temp channels.(not sure why you need a collapsible channel though)

[Linked Image from nodiatis.com]