It actually makes sense for caffeine consumption to change with the switch to vaping (likewise, the feel that caffeine does more).
Caffeine and Nicotine both (indirectly) affect several neurotransmitter systems, including dopamine that you've all pointed out. But Tobacco smoke also contains small amounts of a couple of MAOIs (monoamine oxidase inhibitors), which inhibit the activity of Monoamine Oxidase. MAO, in turn, breaks down monoamine nurortransmitters in the brain, including dopamine and serotonin.
Without taking an MAOI (through tobacco smoke), your MAO system will adjust to working normally and you could see a change in the effects of caffeine, potentially leading to more adverse symptoms at high doses or decreased consumption…or the opposite depending on a lot of factors (tolerance, habits, other drugs, etc.).
Sorry if that went over people's heads…I kind of have a degree in drugs. Note that this isn't backed by research that I've done or read…it just makes sense with how the drugs work.
The complicated thing is that the human brain is really good at adapting to changes in neurotransmitter levels. ADHD/ADD is treated with stimulants, which shouldn't make sense without a lot of other theory behind it. And changes like this sometimes cause cascading changes in drug-taking behavior (still just talking about legal drugs here) that aren't all that predictable and may not be the same from person to person.
As for personal experience, it does seem like caffeine is hitting me a bit harder than it did a couple months ago……but I also intentionally lowered my caffeine intake. I like for my caffeine to give me a buzz, and I wasn't getting it…so I decided to consume less in order to kill my tolerance a bit and not have to drink several red bulls or 4 shots of espresso to get a rush off of it.
I'm not sure which one is more responsible, but I'd guess it's the fasting for a few weeks. I went from several french presses a day to maybe a couple sodas a day intentionally. And yes, those first couple weeks kind of sucked.
Basically, any time you're screwing with brain chemistry…a lot of stuff can happen.
The individual drugs are pretty well understood (with a few exceptions) but there is basically no such thing as a single-drug user. People who drink coffee and smoke cigarettes are getting caffeine and nicotine, of course, but also a couple MAOIs other plant-based methylxanthines that affect hormones and kidney/bladder function among other things, plus all the "crud" in cigarettes…some of which modulates neurotransmitters or other bodily functions.
A change as big as switching from Nicotine + the rest of the stuff in cigarettes to nicotine + the rest of the stuff in juice (that seems to all be biologically neutral) is going to alter the way other drugs work. It's something to be aware of so you don't wind up fighting tobacco cravings and caffeine withdrawal at the same time……but as we all know…it's still worth doing.
