Would adding a couple of drops of vinegar to ketoconazole cream be helpful in treatment of yeast infection on penis? Is vinegar harmful to skin

Not helpful. The Ketoconazole Cream should be effective without vinegar. Acetic acid solutions can be used to unmask hpv induced skin changes prior to treatment, but should not be considered a skin treatment per se.
Complication- Using a cream on your penis suggests the presence of a rash, ulcer or lesion. A fungal infection is unusual on the penis- uness diagnosed by a physician, it is highly recommeneded that you do this -the lesion may be syphilis or other infection that will surely get worse with an acid, and can increase you chances of getting HIV if engaging in unprotected sex.