On MTGO, cards finance is all about supply and demand. When some cards suddenly start seeing more use, it's normal that they go up in price. When they see less use, it's normal that they go down. I'm not sure when it happened, but I was looking at vintage price on MTGO and waiting for it to go down, before there was vintage league. I was almost buying vintage decks when suddenly they announced the leagues, then everyone wanted to get into it, and the prices spiked like 3x. Then as people also realized the prices were too high, they started selling cards, and then the price started getting lower and the VMA drafts were announced and the prices went back to where they were.
When VSL started, prices went up because people saw the games and wanted to get into it too. Now that it's ended, people are less excited and there are very few people playing vintage leagues. It does explain a lot the decrease in prices.
The prices oscilations in MTGO are insane, compared to paper cards, probably because of the bots, and it's a pretty good thing. I currently look up at 4 sites when I'm looking to buy expensive cards. The difference between the highest buy price and the lowest sell price is ridiculous. As it's all automated, they earn only a few pennies with each transaction. When I first began and didn't know about the bots system (didn't know where the good deals were), I lost lots of money and wasn't excited at all of changing decks. Now I'm changing decks all the time, going for a ride in modern, standard, legacy... If you buy a deck now, get a 4-1 in a league and then sell it, you probably will still make some tixes of it.
But of course, if you'd like to just get a huge collection and leave it there, you will lose money, in vintage. Unless you get good results in competitive leagues, then it will compensate.