Interestingly, citric acid (2-hydroxy 1,2,3 propane tri-carboxylic acid) is a major non-volatile organic acid in tobacco. It it completely non-volatile (will not vaporise); rather it decomposes to give some carbon monoxide at around 240C - a temp easily reached in the atty (at least on those occasions when the coil begins to dry.
Acetic acid (vinegar) on the other hand, would vaporise ok; however, it might not be a wise flavoring because the possibility that it could protonate some of the nic to a salt that may or may not be cleanly released on heating.
In general terms, we need the least possible additives (unless something was very safe and somehow increased nic yield; unlikely, but i wouldn't rule it out). If additive X led to less oxidative and decomp products, it would be worth considering; but this is rather unlikely.
So, if the nicotine is protonated to any extent by acidic additives, is the nic is released when the salts are heated. That is possible, though may well be partial. Carboxylic salts, in particular, can perhaps release the nic unharmed on heating. (Leaving aside for the moment what happens to the rest of each carboxylic salt mcolecule, the reconstituted acid).
But without knowing for sure that none of the nic loss is due to acidic additives, these additives are not a good idea, even the fully vapable ones.
Because droplets of liquid are pulled off the metal mesh and some may miss the coil and survive the fiull journey in the airstream (if small and light enough), some nicotine might arrive in the body this way (~1% ballpark).
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Btw, I have a new hybrid design detailed here:
Another novel design for the e-cig / PV - the Hybrid 2 The Hybrid E-Cig 2.