Hi Scott,
Who determines whether consensus on a new chair has been reached?
It are the working group chairs that determine consensus. If a chair is personally involved (whether it is a policy discussion or something like this) then he/she usually abstains and leaves the determining of consensus to the other chair. If someone objects to a consensus decision then this can be discussed in the WG or appealed following http://www.ripe.net/ripe/docs/ripe-614.
If a vocal minority objects to replacing the current chair and blocks consensus on a new one, does that mean he could continue chairing the WG indefinitely?
Consensus is based on discussion. The minority would need good arguments to oppose the change. And if the worst happens there is always the appeals procedure, but I hope we'll never reach that point over who gets to do the work of being a chair :) Cheers, Sander