Seems I'll be the first to give my appreciation to Laura Bailey. Shame that she wasn't brought up sooner. Yes she's in just about everything under the sun these days but it's with good reason. She's one of the most versatile voice actors of current times right now. She did play a handful of characters in NV but my affections for her mostly extend towards Dead Money with the split role of Vera\Christine. Now you can call me biased if you want because I'd basically lump everyone that worked on DM up there in my favorites, but even something so simple as comparing the two radio messages that open and close the add-on, the different feelings that come with each just really show off how excellently she can shift moods and make it feel so heartfelt.
Now that being said, while Veronica Belmont didn't have a ton of lines as Christine's original voice, I did stack her up against Laura and while it wasn't bad per se, I didn't feel the same amount of conviction that Laura's acting gave off on the topic of the Brotherhood in the few lines she also did have on the subject.
I technically loved all of the voice work in the add-on but the last thing I'll say on DM regarding its voice work: The ending slides? Yup, can't even see the last lines written out without tearing up a bit.
Other characters I enjoyed were:
House: The actor really was picked perfectly. House has such a way of being both stern and attempting to appease even if it is in a sarcastic manner. There's something pretty special about feeling a voice booming out of speakers while you merely stare at an image on the big screen. Rather commanding if I do say so myself.
Boone: It's funny to me. He's not a character with a lot of dialogue but once you start unlocking his personal quest, during and especially afterwards, I find myself very fond of his acting. You can tell how truly thankful he is that you help to put his demons to rest all by the tone of his voice instead of a five hour speech.
Cass: She's more or less the flip side of Boone, having a lot of dialogue but her actor pulls it all off so well. Taking Cass around will get you such a nice variety of emotions from sarcastic, downtrodden, creeped out to goofy, serious and thoughtful. Like others have said she feels like a real person and the wonderful voice acting really helped that out.
Probably the VA that surprised me the most was Caesar. I was expecting something a bit... different than what we got. It's not bad per se but as soon as he opened his mouth I thought "Well that's certainly not what I was expecting..." I'm kind of thinking that was the intent though, and if so then well played.