why does baking soda and vinegar unclog the drain?

I seriously doubt that sodium bicarb would have any real affect of solidified fat - plus the addition of vinegar, acetic acid, would neutralize it. Sodium Hydroxide (caustic soda) is the usual remedy - it really does create soap in the presence of fat, touch it and you make your own washing up liquid. It also gets hot and liquifies it. The only action I can imagine that bicarb and vinegar has is the expanding gas clearing a blockage that has previously been loosened by the hot water.

No need to rely on memory or do more reading. Dump your bicarb on a quarter pound of lard and see what happens.

It is in my sink. And every drain I've ever unblocked. You forgot tea leaves, coffee grains, hair, carrot peel and dirt. Maybe, but it'll be slow and ineffectual compared to caustic soda - which is, as you guessed, and I'd already pointed out, is what constitutes old fashioned drain cleaner. Which is pretty much exactly what I said. Maybe. But really the boilng water is doing all the heavy lifting.But why piss about? Caustic soda is cheap and readily available, it does the job without all the hassle of boiling water - it produces its own heat - and is a really strong base. I thought it unpleasant to mention it earlier, but it's so good at breaking down fat that it's used in the industrial production of soap - the Nazis used it to for exactly that purpose but with human fat. Not baking soda.

I'm doubting it would actually have any real effect on a real life blockage. I would be interesting to test it irl.

No - it's a bloody interesting idea. I'm removing my car battery as I type.

Ah, bless you! I know all about how acids and alkalis react and exactly how dangerous caustic soda is. I've messed with them all - and worse - as a kid. I'd say more but it would probably result in a visit from the security services.

Persistence, flexibility and ingenuity.