Is it necessary to add bleach or vinegar monthly to HVAC line when system is not in use, mainly when air conditioning and heat are not necessary.
I agree with ThreePhaseEel, no. If the unit is not in use, then there should be no reason to preventively treat the condensate drain-line.
However, you may benefit from adding water alone to the condensate drain-line monthly, if it's connected directly into the building's sewage system. If the condensate dumps outside or into a sump-pit or sink, then not even water is needed.
If you're adding bleach and/or vinegar to somewhere else, then stop it immediately as you should not be doing whatever you're doing...even if it's a humidifier. Otherwise, monthly to the condensate drain-line is not ever needed and only annually is more than sifficient.
The reason chemicals are added is to prevent mold growth the mold can plug the drains and some kinds are hazardous. If the air handler is not being used you don’t need to continue adding. Just a note I would not use vinegar it is a weak acid and acids will shorten the life of the coils and etch zinc galvanized coating on the sheet metal. But if the system is not being used no additional treatments are needed.