I think you're asking a lot of questions at once, so it might help to follow up and clarify exactly what you're looking for, but I'll try to throw some thoughts out.
For starters I'm obligated to reject the premise that vintage is in bad shape or dying at the moment ... but that doesn't mean that specific metagames aren't shrinking, and I'll take your post at face value and assume that it's getting less popular in your area
Are you trying to maintain the value of your collection?
"the rest of the cards I run should be as applicable as possible in Modern to avoid sudden collapse.
If you're talking about long-term #mtgfinance I think you have things completely backwards. Vintage cards don't really collapse in the same way Modern cards do. If you want to hedge against collapses, you'd try to AVOID cards that are in Modern, not buy into them.
Some Modern staples that come to mind as currently playable Vintage cards are Vengevine and Mox Opal. The risk of Mox Opal dropping is so much higher than the risk of Tundra dropping. The only cards in my collection that have ever gone down in price a year or longer after I bought them are cards that were popular in Standard or Modern.
If you fear that declining Vintage play will result in staples like Force of Will getting cheaper, you can stop worrying. Vintage has never been a real driver of card prices, and hasn't been since at least when I entered the format in 2002. Dealers, speculators, collectors, Commander players, Modern players and Legacy players have so much impact on these card prices that the tiny pool of vintage players never makes waves.
If you're really trying to protect yourself from sudden collapse, investing in Modern staples seems like one of the worst ways to do it, and reserved list cards one of the better ones ... though if I'm being totally honest, the real advice I'd give would be to play proxy vintage and keep your money in something safer than cardboard altogether.
Are you looking for strategies that are good in Vintage now and will be good in Vintage years from now?
That's not so hard, and there are lots of non-finance reasons to be interested in that question, too .... Shops, Grixis Big Blue, Dredge, Storm combo have all been around in some form or another for maybe 10+ years, these are all fairly safe investments from a "time spent learning to play" perspective, though I don't think you're asking about that.
Are you looking for decks that are cheap to get into if you already have a Modern collection?
Totally reasonable question, especially if you're trying to get non-Vintage Modern players into the format. I know very little about the Modern metagame, but I took a quick glance at MtGGoldfish.
Humans as you mentioned is very similar to decks that Vintage players have been running for years. I don't personally like Vintage humans, but it's certainly an established deck, and you could 15 proxy it from a Modern list with no real issues
Dredge in Modern is more than 15 cards away from a Vintage Dredge deck, but the cards its missing are quite cheap. A Modern Dredge player could absolutely buy into the Vintage version easily.
Hardened Scales has a ton of overlap with Vintage Ravager Shops, though buying into it, even with 15 proxies, might require some cards in the $30 price range like Wasteland.
Azorius Control is strategically very similar to a U/W Vintage Big Blue deck, or a slow version of Jeskai Mentor, a Modern Azorius Control player would be comfortable playing the deck, but they may not share enough cards for them to quickly build it.
And while I didn't explicitly see these decks listed as Modern decks, I'm willing to be there's a reasonable Modern deck that's close to U/R Delver, and a reasonable Modern deck that's close to BUG/Leovold Control.
Specific cards that jump out to me as played in both Modern and Vintage include:
All fetchlands, especially blue ones.
All cards with Dredge
Lightning Bolt / Path to Exile / Dismember
Thoughtseize
Noble Hierarch
Cavern of Souls
Walking Ballista / Wurmcoil Engine / Arcbound Ravager / Steel Overseer
Snapcaster Mage
Thalia, Guardian of Thraben
Mox Opal
Teferi, Hero of Dominaria / Jace, the Mind Sculptor
Bloodghast / Narcomoeba / Prized Amalgam / Gurmag Angler
