Vikings on the offensive (i.e. not while the Void Rays are attacking a base) generally tear through Void Rays. If Void Rays are powered up, hitting a base... god help you. Otherwise, Vikings are very very dominant in the air. I've found that my protoss has a hard time dealing with mass mass vikings. Stalkers are pretty good against them but T usually has tanks and MMM dominating ground...
P shouldn't have a problem against Mutas, what with stalkers and the now extremely buffed Phoenixes (which destroy Mutas due to extra light damage).
vikings attack too slow in air-mode. it gives void rays enough time to charge up and destroy them all. for their cost, vikings are a bad counter vs voidrays