But having the vendor devalue your items to nothing makes sense?
If I can steal from them, I will, because frankly they are robbing you just the same by giving you nothing for your warez. You can't even buy a helmet without selling 100 things for 1 - 10 gold each.
The barter system is a joke.
But the problem is that you shouldn't be able to steal three sets of armour from their pocket, not that you shouldn't be able to steal from them at all.
The simpler solution would be for them to have in their inventory just the key for their stash and maybe some gold or small items. Then, after you stole the key, if you want to rob them you just need to open their secret stash.
It's not that easy to steal the big ticket items. The small stuff (health pots, scrolls, arrows, etc.) is easy (too easy) but for armor (or even things like specialty flasks) you are basically looking at 1/2 hour to an hour depending on your greed or patience level of quick saves and reloads because the probability of failure is pretty high. It actually takes a good bit of effort to set it up so that you get away with just one big ticket item without having the vendor and the entire camp go hostile.
Also if you steal the gold you can't double dip and sell to them because they are out of gold (since you stole it from them). Knowing I can steal the gold ahead of time allows me to bypass having to pick up garbage; avoiding inventory clutter and only grabbing things I think can be used for crafting down the line.
I have always played some sort of a spell caster because I've found the other classes to be boring. Gearing without an effective thief however was always a pain. This time I made a thief and while I can't have any fun with spells, I can raid a vendor and plan a heist making him a lot more useful to the party and - above all - actually fun to play with.
I would agree that some things are too easy to steal and may need to be looked at, but armor and more expensive items are not as easy as this thread may have you believe. I would agree with all of your points here if, like I said, the vendors did not rob you all the same by giving you nothing for your gems and jewelry that you got excited for when you found it out in the wild.