Actually, I can tell you for a fact that it does. Namely, my gripe is that whenever I get an E-mail over the Outlook app (for work), it stops my music (right now, I'm listening to southern gospel station KWFC from Springfield, MO through their own app). However, I also have Grandfather Clock installed for Westminster clock chimes every 15 minutes, and while that introduces a split-second hiccup (maybe about a quarter of a second or so) in the music when the chimes ring, I can hear the music and the clock chimes at the same time (it does, however, interestingly enough, disable my bass boost when they ring, but re-enables it after they stop ringing -- that I can live with, at least).
Like you said, there's vibrate, but that's not what I'm looking for here, as I still want to hear the notifications. I just don't want them killing the music and forcing me to restart the player.