Not sure if this is actually Dawnguard or Hearthfire related, because it happened on a Dawnguard quest, and the bard is Hearthfire related.

I was on the mission to turn my spouse into a vamp, which I did (in Windstad Manor), but apparently my bard saw this happening and ran off, then just vanished from the game. I never actually noticed her in the room while I was turning my spouse, and after several saves later noticed she (the bard) was gone. Left the house, waited 11 ingame days, went back to the house, and still no bard. Tried clearing the cache as well and the bard is gone. Also no option from my steward to rehire one either.

And why would the bard ever come back? You attacked your spouse, maybe the bard thinks she will be next? Better be more careful next time to make sure no one sees you having a snack.

Likely the bard will eventually turn up, I doubt they actually have AI smart enough to consider self preservation above the job.

It is also possible the bard was killed when fleeing or like previously stated she may eventually turn up.

Nice idea, but I would assume the game mechanics would move the bard somewhere in the game. It's not so much a question of are they gone but where have they gone...