Your experience doesn't necessarily mean Vocal is on its way out. Plenty of writers do quite well there. My previous works all earned. And while $3.80 per 1,000 reads (for non + writers) seems low, it's for ALL reads, no matter how the reader landed on the article. One of my pieces was shared by a company's CEO and receive 20,000+ reads in a span of hours. I got paid for all of them. Each platform is different, and there's no one-size fits all.

On thing to note: if you're copying / pasting from Medium onto Vocal, you likely will see less traction on those works, especially if Medium already resulted in a good Google Rank. That's SEO, and it's a good reason to introduce fresh perspectives across platforms.