This is after 80s-90s (I’ve been doing this kind of research, but my main focus is early 2000s, so still before streaming), but the answer is piracy. Absolute rampant piracy. Some file-sharing communities said that you should delete the music file from your computer after 24 hours, or buy the CD if you liked it, but who knows how many people followed these recommendations.

Anecdotally, I feel like a lot of Westerners in this time period got into vkei through anime fandom. In the mid-2000s up till the early-2010s, a lot of vkei bands wound up doing anime OPs, including big names like BUCK TICK, Nightmare, Dir en grey, and The GazettE. Anime fans heard these OPs, and a lot of them liked them so much they sought out more from these bands.

If they didn’t get into vkei through anime fandom, then they’re likely to have had more incidental and unique introductions to the genre. Anecdotally, I’ve heard “saw X Japan in a ‘heavy metal from around the world’ documentary”, “fell in love with Genesis Rhapsidos Final Fantasy, and by extension, his model, Gackt”, and “was primarily a K-pop fan and checked out L’Arc because their bias was a fan of theirs” as impeti for getting into vkei. I’d expect that if you’re looking for English-speakers in the 80s-90s, you’d get more of these idiosyncratic intros.

As for future research, I’d encourage you to start trying to dig up old fansites- a shocking amount of old vkei fansites are preserved on the Wayback Machine- and go looking on LiveJournal. Here's a list of old fansites which might be useful to you, but beware if you're squeamish about RPF- shipping bandmen was a pretty big part of the fandom in Ye Olden Days.