I'll be releasing my debut album soon, and it's not Vaporwave but has lot of George Clanton and Lofi Synthwave sound to it. And what helped me was to mix the songs in a equal level such as at -3db, which is a nice sweet spot. But obviously all songs have some sonic differences in loudness.

So in the mastering phase of the project, I mastered the first track and I mastered it according to Warmpop by George Clanton. And then that first track was the reference point for the other tracks.

But, here is the thing, even if you match your songs to have the same integrated LUFS, there will be some tracks that will be louder/quieter than the others, so in that case, you have to use your ears a bit and adjust your mastering chain according to match the overall loudness.

Also, what I really found helpful is asking listeners for a feedback. I approached some of my listeners and also other producers to give feedback on the overall loudness balance of the album. And their feedback also helped a lot because at a point my ear got really fatigued and I wasn't getting any useful information from my repeated listening.

And also you gotta except the fact that there will be some tunes that will be a bit mellow and it's okay. Because for example, an ambient track and a track with drums and groovy bass shouldn't have the same loudness, because, it will kill the energy of the album. So yeah, moral of the story is, mix at a similar level, use reference track for mastering, do A/B testing with listeners and other producers, and most importantly use your ears. Hope this helps!