I always thought it was combined.

It is combined

Which pretty much negates the advantage of aiming for the head in Fallouts 1 & 2.

What about the extra damage?

MAIN advantage- having less DT to punch through

I'd say dealing double damage is a bigger advantage then ignoring DT when your using the higher powered weapons in this game. Helmets made for combat can be just as effective at absorbing or deflecting bullets as the armor its made for. (of course if your helmet fails your more liklely to die then your kevlar.

Only of foes in the game will survive a head shot from one of the top tier weapons, few still will survive a critical to the head. Nothing in the game will survive a sneak attack headshot from the anti-material or gauss rifle on normal difficulty.

In Fallout 1&2 headshots didn' ignore armor if the targets armor came with a helmet. (armor being a single item in those games). The main advantage of the headshot was the extra damage and a critical was more likely to cripple or outright kill the foe. If the enemy actually had decent armor it was armor that came with a helmet.

Double damage? I see. I was thinking a 1.1 damage boost.

I don't think it's combined. I coulda sworn that on high-DT enemies, if I aim for the head, my damage always got significantly better.

Because head shots deal double damage.