Unfortunately I'm at work and I can't go into the game and try this out right now, but I think I've done exactly this thing. I bring up VATS, click on the area(s) I want to hit with my mouse (same as hitting E?) and then press the spacebar and... nothing. Again, I'm sure there's something obvious I'm missing.

http://www.nexusmods.com/fallout4/mods/531

No, but bugs in a UI display are nothing new in games. I KNOW that I've had higher damage hits from Iron sights and (mentioned above) some number of my laser hits generate piles of ash from iron sights hits. Again, I see no evidence than my experience to convince me you cannot get a critical except via the VATS Crit bar.

No, that's not how it works. Clicking on the area alone doesn't execute the attack. You can select multiple targets inside of VATS before actually executing the attack.

What you need to do is hit Q (or use the mouse) to select the targets. You can also use W and S to cycle between regions on the same target, and A and D to cycle to a different target. Then hit E to execute the attack on the targeted sites. To execute a critical attack, you have to hit the spacebar right after hitting E.

I'm on console, but I love the new critical mechanic. I use the banking perk and VATS and the perk that randomly fills by crit bar and it all stacks very nicely.

I like it too. It means you can save up your critical attack for when you really need it. That's better than it happening at random.

I love VATS. I shoot all the legs off ghouls with VATS, then I use my ripper to finish them off.

Agreed. Utterly.

So ok, how do you charge up and use a melee 'power attack' in VATS? Is it possible? Or do you only get normal melee strikes in VATS?

Okay! This worked! Thank you very much. However, I have to say that was not at all intuitive and they only give you about one second to select crit. If they aren't making crit random like Fallout 3, it seems strange they would make it so difficult to execute.

Can honestly say in 2 1/2 solid playthroughs, I can't remember ever using the critical hit thing. I prefer to shoot them in the head from a distance. Works for me.