Sorry if this is rude. How old are you? How did you get into vintage shoujo?
This question is hard to answer... Here are the thing I think that made me partial towards older shoujo.
As I was growing up in the 90s (which also shows how old I am), we had a lot of old anime running on our TV stations and obviously, I watched them. I was especially crazy about Candy Candy.
Fast forward to 2007: After I became a weeb through other means, I realized that many of my favorite childhood cartoons were actually anime, and I wanted to re-watch them. Candy Candy in particular, of course. Which is when I found out that our TV station had only aired 1/3 of the anime (it stopped when Terry left for the US in every re-run). So I sat down to watch it from start to finish. Those were very happy times for me.
Mawaru Penguindrum-Shoujo Kakumei Utena-Rose of Versailles pipeline got me even more interested. As MPD was airing, everyone urged me to watch SKU. And when you watch SKU, you are obligated to watch RoV, the original épaulette lady. I was in shambles after watching RoV. I was pretty much convinced that the real good stuff must have been in older manga.
Becoming a fujoshi helped. I was lucky enought to have a friend who recommended me Zankoku Kami ga Shihai Suru when I was first discovering BL. That manga made me develop an academic interest in BL, because I wanted to know how something so peak could have come to exist. The more I read up on the history of BL, the more I learned and got interested in the bishounen and shoujo manga culture.
Reading Kaze to Ki no Uta was the final push I needed. The emotional support I needed after finishing it made me meet other older shoujo fans, and things escalated from that point on. Quickly.