Well "Vats" in Fallout is different because you're essentially pausing the game, so you don't have to be able to move whilst pressing it
As for the Metro: Last Light, like I said, without headphones, I could barely hear a thing, and although there were sub-captions for the voiceovers, I wasn't actually paying attention to it.
I'll try and give a brief rundown of the demo:
It starts underground (duh), in what seems like abandoned sewer shafts. You walk forward and meet up with your friend...he needs a boost to get into a pipe, so you lend him a hand and suddenly he's grabbed by the enemy.
So then you go hide...I'm not sure if the audio would have prompted me or not, but that's what the NVIDIA rep told me. She basically said, "your friend was just grabbed and they're going to come looking for you now, you may want to find some cover." That's when she also said to use "V" to stealth melee kill when they walk by.
So I go hide in a crevice behind a bunch of spiderwebs (which looked great in 3D). A big door opens and out comes two guys searching for me. After they pass, I sneak behind them (I crouch-walked, but not really sure if it was necessary...without audio, couldn't tell if that affected anything), and then pressed "V" behind each, treating me to an animation of my hands reaching around the guys head and silently slicing his neck.
After killing both, she told me that one had a shotgun, so eventually I found it and picked it up. Simply pressing "E" on the guys picked up ammo, but for picking up the gun, I actually had to search the ground for the gun itself, which was slightly annoying, but at the same time, more realistic.
Next I went to enter where that big door opened. She said I could turn off lights, so I went by one and did so...nothing fancy.
Inside the door was a big factory-like area. Since I had the shotgun, I wanted to see how some non-stealth combat would be, so I bumrushed and started pounding guys in the chest. For the most, one shot to the chest killed them. It's possible I simply missed some and that's why it seemed like it took more than one, or maybe I only hit a arm or leg or something. It became a little chaotic after the first kill, as there were about 5 guys attacking me at once. It wasn't a tactic I'd normally use in a FPS, but hey, I was here for a demo, so I said eff it and just went nuts. I switched back to my other gun, which I couldn't really tell what it was...maybe a machine pistol.
After that, I moved along and there were three more guys that just entered the area. She told me I could shoot this butane lantern and start a fire, which would also create smoke for some cover. Before I could even do so, they spotted me, so I picked off two of the guys. The third was behind the pillar, so i took the opportunity to shoot the lantern just for kicks and the fire effect was certainly sweet, though I didn't notice much smoke (maybe I didn't wait long enough)?
After killing the third guy, I enter a door and there's a guy laying on the bed. He starts to move forward and plead for his life...I shoot him in the head
I then climb into a sewer pipe and crawl my way through. There were these mutated spiders that scurried about every now and then, but I could perform no actions other than walking. I eventually get to a grate and my friend is on the other side. We exchange some words before a guard enters his chambers and takes him out.
That's where I stopped. I'm not sure how much more the demo goes for, but I don't think too much longer because when I first got there, the guy before me left it at the point where the guy is pleading for his life and the rep said it was near the end of the demo, so she restarted it for me. She didn't tell me to stop playing, but I didn't want to be a dick and just keep playing forever. I had seen enough to experience the game and appreciate the 3D in its current state.
As Jim said, it's quite possible it's not fully optimized yet because it's practically a beta. The rig was running dual 690s! Graphically, the game certainly looked "finished" though.
So the mission in the demo is certainly that of a search and rescue type of thing, but whether that's the whole game or whether that's just this small section, I don't know. I also don't know if this was the very beginning of the game or if it occurs somewhere in the middle. Stupidly, I forgot to ask that of the rep. I will say that there were absolutely no prompts to help me, like telling me what to press, so it certainly wasn't a "tutorial section".