By default, as you've probably noticed, channel names are sorted alphabetically.  Thankfully, you are able to override this using either 1) raw charisma or 2) admin functionality in the Ventrilo client.

1. Log in as an admin
2. Right click in Ventrilo and choose "Server Admin"->"Server Properties"
3. Change the "Channel Ordering" from "Sort Alphabetically" to "Manual"
4. Save it.

You should now be able to drag channels around to change how they are ordered.