Lets say that the Smith Filnjar at Shorse Stone outside Riften is killed (he is just a unique character not essential or protected or will not respawn), will there be some other NPC that takes his job and place?
A bit sad if all vendors that isn't living in a Hold just get killed and no one else is taking over, then the world gets even more empty then it already is.

Try it and see...

Well i have limited play time due to REL, so i don't want to waste it on play testing
Thats why i asked if some one did know this, so i can correct it my self if no one takes over by making the existing vendors cautious and not help any one

It seems to only work with particular NPCs. Killing the old owner of the Alchemist's shop in Markarth results in her young apprentice taking over ownership of the store.

This isn't true for a lot, if not most merchant NPCs though. At least in my experience.

On one of my evil characters I'd just finished training my alchemy as high as it could go with Arcadia. Being evil I opted to not pickpocket my gold back this time and instead murder-kill-deathed her to get my money back. After merrily skipping out of town with the blood of an innocent shopkeeper on my hands, I returned to Whiterun after some time away to find that she hadn't been replaced and her corpse was still on the floor of her shop. Being all corpsey, y'know doing corpse stuff, the things that corpses do. She was still all the different kinds of dead except for the 'un' kind is what I'm trying to say.

Whether or not I didn't stay out of it long enough for a reset to occur or Arcadia simply doesn't have anyone to take over her shop, the result was the same. I was minus one Alchemy shopkeeper with no replacement in sight. I get the feeling the same is true for most shopkeepers in Skyrim, some will get replaced but probably only those with people you can meet to take over.

OK...well i have made a quick fix that makes all non essential vendors cautious, so they don't take over soldiers/guards job to protect and serve!

I smell an idea for a mod.....
Correct me if i'm wrong, but I believe only certain named NPC's have replacements and All generic NPC's Respawn. I wiped out all of whiterun's guards then come back a week later to find about 15 more there trying to beat my face in.

Well I'm releasing the small tweaks i have made for vendors on Nexus soon, surly it's more then me who thinks it's weird that ordinary vendors was sometimes even more
brave then any soldier or guard around?...they are shop keepers not warriors!