I finally got Voice of the Vampire and have been using it in the trials. So far, I've had very few resists and charm seems to go the distance. Overall, Vampire works very well here. Added dps and so much more efficent then mezzing.
In Fear, the fearless can be mezzed and charmed. Helps a lot when you get three or four fearless in one wave.
In Efficency, the soldiers can be mezzed and charmed. I get frustrated when asked to keep two soldiers mezzed, because I can't do much more, except try and pulse mana song. With charm I could keep one mezzed, melee and twist still. The only difficulty was getting the pet retargeted on breaks.
In Power, some of the mobs are charmable, others not. I did charm a girplan, but got non-charmable messages on others. I'm not sure what the pattern is. I haven't even tried mezzing in power, usually just use AE slow.
I haven't charmed in Subversion, although I know the two guards (not the patrollers) are chanter charmable. It didn't seem worth it to me to try.
In Inguenity, the dragorns that move off their pedastal (vs the ones that stay planted) are supposed to be charmable, but I haven't tested it. Now that they've changed this one to DT pets, pet classes are hosed, but that might make it better for us. It's likely to be dead before charm breaks.