You can vaporize ethylphenidate, but if it's in a water solution it will hydrolize to ritalinic acid and ethanol. The rate will depend on pH and temperature. So try and keep the pH neutral, don't boil solutions of ethylphenidate in acid or base for long periods of time, and don't store them as solutions for more than maybe a week at room temperature.
The salt form doesn't matter much for oral usage, however different salts will have different boiling/melting points and some might decompose where others will vaporize cleanly. Generally the HCl salt is best for vaporization... organic salts like the citrate will probably burn/char instead.