It doesn't need to be a rental unit. Just an empty spacey that could be a rental unit. Could be a house.
I've seen many houses sit empty for years before they are remodeled and put back on the market for rent. It happens all the time. If you buy it at auction it doesn't hurt you much to take your time to get it on the market. So large companies buy up what they can and many sit until they get around to fixing them.
There are 16,000,000 empty units in the US and only 500,000 homeless. Get all of them in homes and there would still be over 15,000,000 empty units.
That doesn't even include converting office space into residential.
Just need to change up the incentives to get more homes into the hands of people and make sitting on vacant space more painful.