Or an alternate strategy if your goal is to have a vacation rental paid for is to buy some sort of investment, STR, buy and hold, stocks, bonds whatever and just dedicate the cashflow from that to renting a place each year. 

lol  i would not surprise in the future if state gov. make new law in CA saying that using SSN for housing / rental is an act of racism haha and LL has to treat everyone equally (DEI).In theory it's hundred times safer to rent the place to (illegal) immigrant rather than US citizen that has 800 credit score but just get out from jail.

(which usually comes from hands experience) then I think REITs are a good option ... much safer than going out of state investing in individual properties as an inexperienced investor IMO.

They suggested an alternative method, but I would need to provide more information, like passport, which I refused.

Alternatively, consider partnering with a cash buyer or exploring other investment strategies like turnkey rentals.

Alternatively you can simply wait a month or two as the person will likely start relisting the property on those websites once they feel like its safe to do so. 

When making a deal, ask for a bigger down payment to be safer.

This would permanently damage the window, and so I'm looking for an alternative.

However, this option is considerably more expensive, typically costing 5 to 15 times more than the alternative.

A private loan or rich uncle is another example.The alternative with a mortgage is that you have to BRRRR sequentially, not in parallel.