@a1batross
Android tends to have problems with playing multiple audio streams. At best I've found apps able to lower volume for a notification, and then keep playing normally afterwards.
As for killing the app completely after it was paused, that's a low ram issue.
@a1batross
That's odd then... have you tried multiple music apps? Could it be a bug with that specific player?
@a1batross
Only other good music player I know is Poweramp.
Odd that it happens with Vanilla. I have an old, 1.5gb ram phone, that I mostly use as an audio player (music/podcast), and I've never had such issues.