Forget the bus Phuket to Hua Hin even if it's supposed to be a VIP one. I've done it the other way and the first 6 hours was VIP bus to SuratThani, then an hour wait, then 4 hours on a rickety old bus with windows open to Phuket town. We were not told the VIP bus wasn't going all the way
Save yourself some grief and fly to BKK for 2000baht or less and then a 2000baht 1.5 to 2 hour taxi ride to Hua Hin.