I personally really dig Connery as Vaughn. His lines in alot of the books really translate well if you read them with a Connery accent.
BD Wong is a sensible choice for Shar. Watching a few Law & Order SVUs is all I needed to sell him as the Andorian. He has a great voice and delicate features that can range from very sad & compassionate to stony & angry. All ranges that an actor would need to play poor Shar.
For Sam Bowers another Law & Order actor, Jesse L. Martin.
Taran'atar: 5 Time WCW Champ Booker T
Prynn: I haven't found a good choice for Prynn yet.