Multiple domains, same forum, seamless login?
 

Okay so I'll try to explain my issue as detailed as possible.

I have two domains. Let's just call them
forum.mydomain.com
and
supergames.com

Here's the thing: The forum is ONLY at forum.mydomain.com, BUT I want it so that you can use the login from forum.mydomain.com also at supergames.com

I've already included the vbulletin core files and I can work in the code with it, but the problem is (you probably already guessed it) the cookies.

I can only log in on forum.mydomain.com, even though everything is properly set up in my code.

I've come across this addon for vB3 https://vborg.vbsupport.ru/showthread.php?t=211100

which seems to be exactly what I need, but I have vB4 :/

Could someone please help me out here?

I don't think you can use vBulletin on two different domains with a single license.

Anyways, did you try to use that 3.8.x vB mod? I tried one or two 3.8.x mods on my vB4 forum, and they seem to be working correctly. I mean, I know most mods that are for 3.8.x wont work on vB4, but you should try anyway. Just be sure to backup your vB folder and database before you install it, just to be safe.