The only person I told what MUGEN was about was my cousin because I knew he would understand, trying to explain the same to anyone else in my family would be a waste of time since everything I do on my comp is considered "playing a video game".
Duracelleur's mugen multiverse screenpack is great, it's inspired by the IMT screenpack but it looks better and it can support a large number of characters
MajinVegeta.def will also link to another SFF with different small and big portraits.
You're going to make another sff file just for the sake of 2 sprites? I would suggest you add the two at the end of the original sff and make a small tutorial in the readme for those who want to split the character into two (namely Vegeta and Majin Vegeta).
The common way to install screenpacks is to just replace the files existing in your mugen folder with the files of the screenpack and make sure you don't replace your "chars" folder.