When I visited Riften the other day I noticed that there were four or five NPC's lying dead in the market square, including a vampire and a hound. Among the death was Balimund who is the town blacksmith. I also noticed something else. Some young lad replaced Balimund as the Riften blacksmith. I never witnessed before that NPC blacksmiths or merchants are getting replaced. Is this common, or is it part of one of the DLC content packs?

Most merchants have always been replaced.

It's just more common now that vampires can easily slaughter them.

Yes the fanged beasts slay all the good towns folk just think what would happen if the Vampires teamed up with the Dragons they would kill all quest related characters and shop merchants in Skyrim leaving you with an empty game with nothing to do. Madness!

Yes, the majority of merchants have a replacement, however, some of them don't work correctly. Belethor (in Whiterun) is one of those NPCs who never gets replaced on the vanilla game (without the Unofficial Patches). With Balimund, his apprentice (Asbjorn Fire-Tamer) will take over for him, which is what you have experienced. Be careful though, if his apprentice is killed, there won't be another replacement after that.

Its good that Beth implemented that in to the game. However, I do remember Alvor the Riverwood blacksmith getting killed by a dragon in some of my previous playthroughs, but i never saw him being replaced. Is it because Riverwood is just a small village? I believe Sven would be a good replacement for Alvor. A flute playing blacksmith.

You can pretty much stop the vampire attacks if you stop fast travelling at night.

In pretty sure the smith in riften only gets replaced because he has an apprentice there none of the other cities have that option.

It's very easy to prevent town killings . If theres dead vampires then they were killing town folk while u were there ( cant speak for pc mods)

Not just pretty much, but fully. Vampire attacks do not spawn during daylight hours, so not arriving at night is a sure fire way to never lose an NPC.

Actually i'm pretty sure the only sure way to prevent vampire attacks is to do the main quest of dawnguard. iirc Awakening quest being active fully stops all invasions unless you've sided against the vampires

All vampire attacks on settlements stop completely when you complete either side of the main quest, unless you cause an eclipse of the sun.

However, vampire attacks require certain triggers to spawn at all, one of them being that it must be night time.

Alvor's apprentice is his daughter (kind of), who is a kid, so there is no replacement if he gets killed.