I'm sure they were probably a bit nervous as well. They had to do a special cut of the song for TV and were probably rushed into doing it with little time to prepare. I know what you mean about the vocals being a bit off, but I've seen other live cam vids of this song and they nail it. They probably just had an off day. However, their playing is great.
Also, if I'm not mistaken, this is something kinda new where all of them do the vox, right? I remember their other albums sounding nothing like this one, so they might still have trouble doing it live sometimes.
As Kevin said, the bassist does look like he's struggling, and that might actually be the case. Many people think that playing bass and singing is easy, when it's really not. You have to follow the drums and sing with the guitars, which is no easy feat. I'm glad I'm singing and playing guitar instead of bass; it's not easy but I'm pretty decent at it because I can follow the rhythm and melody while I sing/scream. Hell, that's another thing. Screaming and playing is completely different from singing and playing.