tstcvap-er, that's a heck of a question.
Terry v. Ohio grants officers great leeway with regards to what is "probable cause." I think we both agree that this officer was within his rights to investigate what Grinch was doing.
Let's say that, while questioning Grinch about his e-cigarette, he smells the odor of intoxicants on his breath. While the initial investigation into his vaping turned out to be nothing, the officer now has a right to investigate further into whether or not Grinch was drinking. In short, the officer now has probable cause to investigate for OWI. It does not matter how the traffic stop was initiated; people get stopped for registration issues and end up with OWI arrests.
There is another angle to think about as well: officer safety. If at any time the officer feels his safety is in peril, he can order Grinch out of the vehicle and search anywhere within Grinch's reach; under the seats, the dash, center console, back seats, and, if you have a hatchback, all the way into the storage areas. In some cases and some jurisdictions, the officer may be able to order you to open locked compartments such as the glove box. He is also within his rights to do a cursory pat down of Grinch. This search is supposed to be to locate weapons, but if contraband is found it is admissible under the principle of "good faith." Some officers try to push the limits of "good faith," but honestly it all comes down to whether or not the officer can articulate his logic for doing the search.
But let's try your scenario. If the officer were to pull Grinch out of the car (which he probably would have done anyway to have him explain the e-cig), and then search the car with the expressed purpose of locating contraband, that officer will likely have a problem. Grinch had already explained and proven his behavior was legal; therefore, the probable cause to investigate further ended at that very moment. Any further searches would be illegal; anything the officer found would be "fruit of the poisonous tree" and would be inadmissible.
Just remember, consent overrides all of that; if you give the officer consent, he no longer needs any probable cause to search.
However, I'm not all impressed with Grinch's officer; he may have acted within the law but he should have kept his opinions to himself; that was highly unprofessional.