[SAJ]

Ronn,
You are forming iron acetate with the vinegar, which is dilute acetic acid. Metal acetates are soluble in water and so will dissolve in the vinegar solution. Any residues left will Wash off with water.
You are then left with a "nascent" (literal meaning "at birth") iron surface, which will flash rust rapidly.
So a great thing to do is paint it after the vinegar treatment with phosphoric acid or a phosphoric acid-based product like Ospho.
This forms iron phosphate, an insoluble skin which is great for painting or powder coating.
You probably know the Model A body steel was treated in this way when made.
The flash- rust reacts quicker to form a good phosphate layer than the raw "nascent" iron itself.
Chemistry and chemical names simplified in the above!
Apologies if you already know all this!
SAJ jn NZ