If it's already been covered, apologies but I am a retired Law Enforcement officer of 27 years service. Yes it will take some of your time but one thing (at least for now) we still have is the presumption of innocence even on a civil traffic infraction. The difference is simply that the violation for which you were cited doesn't carry a sentence of jail time in itself.
If the officer was alone, it should be as simple as showing up and saying the officer was mistaken and that you weren't using a cell. By Statute, the officers word doesn't carry more weight that yours. If he is the only one saying you did and you say you didn't, the court then has no proof to find you in violation. He may be a "trained observer" but that is not really applicable in this situation.
Be sure to take any cell phone records (or bill since it hasn't been used in months). Accessing your cell records online is easy if the date to appear is before your next bill is received. The biggest thing is to carry your PV you were using to court with you.
If the officer used a specific statute that applies to cell phones only (as does the Florida law), he has boxed himself in. If he cited you for failure to use due care while driving, it can be a bit more difficult. Any actions a driver is undertaking other than driving can and has been upheld as inherently dangerous.
Dripping and driving, changing a carto or anything that takes your eyes off the task of driving can be considered failure to use due care. I'm not saying that theory is correct but you need to be as well prepared as possible so be sure to state that your use of the PV didn't involve actions that took your eyes off the road. Normal vaping wouldn't do that - adding juice or anything like dripping or the like can be construed as a voluntary action on your part that hinders your ability to be aware of your driving.
Good luck and I think you should win it. Just make sure you don't mess yourself up with comments about anything that distracted you from driving. I might even go so as far as to relate your situation to using analogs. Smoking and driving aren't unlawful in and of itself and vaping is really no different.