[Paladin2]

"a vapor cloud coming contact with a spark from an engine that was idling (fire truck, of all things)"

Spark from a Diesel engine?

[topher]

The exhaust (from the valves of the engine) would be a fire of sorts.

A diesel has combustion from compression not from spark, but it still has combustion.

And a cloud, from what I understood of the explanation, would have enveloped the whole vehicle (my understanding).

I need to find the article that I read that from...

[topher]

This article claims that diesel engines were the source for other refinery fires:

Probe of Richmond refinery fire looking at spark sources

From that article:

The investigator said diesel-powered trucks and generators have been ignition sources at other large refinery fires, including at BP's Texas refinery explosion that killed 15 workers and injured dozens more.

Depends how credible this source is...

[IMR 4350]

If a diesel engine is running it will suck in the vapor and cause the engine to “run away” just like spraying starting ether in the engine while its running.

You end up with a flame coming out the exhaust.