I had some spice resulting from STB + re-X.
I mixed it with vinegar, and then warm water, for the purpose of injection.
A lot was left in the spoon, so I put it away somewhere undisturbed and forgot about it.
Some time later, I ran across it again. I scraped it, and smoked it. Wow.

What exactly was the chemical form of what I smoked? Was it fumarate? And, why did it appear to be more potent than what I usually smoke? (It wouldn't be a question of deterioration over time, because it's never around for long.)

I don't think you should be trying to inject this DMT + vinegar (and whatever impurities/additives it might have). Please read this:

https://wiki.dmt-nexus.me/FAQ#Can_DMT_be_IVed.3F

As for the form of DMT, vinegar contains acetic acid so you tried to smoke DMT acetate, which can work but it can also be irritating to throat/lungs and hence not recommended. Freebase should be used for smoking, salts for oral ingestion.

You say it seemed more potent, but there are a lot of variables there.. What was the exact amount you smoked vs amount you usually smoked? What was the technique used? etc etc..