So this forum seems like it's pretty much dead nowadays, but hopefully someone will see my thread and answer.

Basically what I want to do is create a Perk that gives guns a VATS attack like the melee weapons have. A good example would be the 'Fore!' attack you can use with the 9 Iron. The thing is, I want this to only work if you have the proper Perk, instead of the skill requirement the melee attacks have.

Can this be done, and if so, how?

You can create a perk adding the ability victoryrifleknockdown effect (the same used on iron 9).
Create a formlist with the weapons you want the effect apply
and in the perk condition isweaponinlist and choose your formlist AND getvatsmode==1

I don't know if it will work, but it is worth a try.

From the sound of that, wouldn't that simply cause the shots to always knockdown when shooting in VATS?

I've actually wanted to do something similar to this, and add extra VATS attack options to melee weapons.
In either case, the important thing to find out is whether or not it's possible to add extra VATS attack options to the game at all.

It is probably easier to add VATS attack directly to a weapon than to create a perk to add them but I never used VATS in game, so I don't really know how the iron 9 works and if the fore option is selectable by the player and I don't actually have a character with the required abilities to try.

However, I was wrong in my first answer. It doesn't work using an ability but with an entry point it works
It always cause a knockdown but only with the weapons in the list.
In the condition for the perk : Getvatsmode==1
Perk entries : entry point - knockdown chance - set value 100
conditions : isinlist formlist knockweapons==1

I created a formlist with the 10 mm pistol and the varmint rifle. Both knocked the test NPC. The 9mm and the hunting rifle didn't.

The percent chances can be adjusted with the set value, to give a "random" percent chance for a knockdown. but I don't know how to make the knockdown effect directly selectable or not by the player in game.
The best I can think now is ranged weapons allow the player to aim at a specific bodypart. Then the perk could cause a knockdown only if the specific part is shot and give a normal shot if aimed elsewhere.

The option is selectable, that's the point. When using vats, you can press one button to queue a normal attack, or a different key to queue a second type of attack when using a melee weapon.
For example, with a dagger, you can either use an ordinary attack or you can use "Back Slash" which has a unique animation and does less damage in exchange for costing less AP.

It's not so much the effect itself I want, but rather for the option to show up (eg. "Fore!" or "Lights Out!") and for the animation to play when selected.

Basically, what I want to do is make pistol-whipping possible in VATS, or by doing a power attack (which are basically the same thing).

Tried again with a melee weapon. I suppose it is not a bug if you can't aim a specific bodypart with a melee weapon. Didn't know that.
So the effect would be much easier to obtain than the option to select it, if this option is possible to add.
Let's hope some geck dinosaur will resurrect to come on the forum and can provide a solution.