Ever bought something on Steam and have it simply vanish from your account?

No.

That's interesting. I've seen things disappear from libraries because of a stolen key or credit card or something, but never from an inventory when it's bought with wallet funds.

That is weird. I've never run into it myself, and the only possible explanation I can come up with (aside from just being a bug) is maybe the wallet funds came from an illegitimate source? Like, maybe you sold something on the market that turned out to have been stolen or bought with a stolen credit card by the person you got it from, so once they noticed they simply wiped out the value of the sale? But in that case I'd expect your wallet would be missing the value of whatever you sold, rather than exactly the $3.74 that RCR cost.

i only lost coupons so .. i was the lucky one of us i think :D

Not yet, but posts like these always make me want to run back to my wins list and ensure everything is still there.

New development: Steam no longer has my credit card information on file.

no

UPDATE: I decided to buy something else, and after that purchase was complete both it, and RCR appeared in my inventory.

Next time this happens just trade anything, with someone. It a glitch of the inventory to 'hide' the latest acquired item from time to time.