well VATS is awesome but FO3 was easy because of the too many skill points, too many perks, too many SPECIAL points and not enough high firepower weilding enemies, even at level 20 or 30 raiders still had lead pipes and not enough other enemies had high firepower, VATS wasn't the reason FO3 was easy. but as far using VATS, it all depends, sometimes i use it sometimes i don't, sometimes not using it is more efficient and sometimes using it is more efficient it just depends on the weapon and situation etc.
You are right that Fallout 3 was easy because of many factors but wrong about VATS not contributing to that. Its essentially an I WIN button that can be used in every encounter. When you use VATS it doesnt matter what weapon your opponent has because you are nearly immune to it anyway while giving out nothing but headshots.
An aimbot that reduces all incoming damage by 90% and increases your crit chance certainly makes the game very easy.