There aren't long term studies on vaping because it wasn't done en masse until recently.
This is all anecdotal. Not even an educated guess, just a guess, but I think that "vapes" actually span the spectrum, from "worse than working in a coal mine, unmasked" to "less physically detrimental than a heavy sugar/overeating habit", based mostly on 4 factors:
1) coil/atomizer metal type and quality
2) quality of juice/distillate/what youre vaping
3) temperature of vapor coming out of device
4?) User input- do you rip it right to the lungs, or mouth-inhale? How long do you inhale continuously?
Vaping certain things could definitely be worse than smoking. I have had a long career of smoking and vaping many things, all-day-every-day style (not proud. sorry, body) and just from the surface, the hardest on my lungs was vaporizing weed products, whether it was carts or dabs. Far worse than cigarettes, even. Taking giant rips from carts (10-15 seconds) fucked my lungs up bad, which probably shouldn't have been a surprise lol
The one that seemed least damaging was the old school nicotine vaporizers with the giant coils and giant mods. They use different (higher quality) metal for the coil/atomizer, which is a key part of bad shit getting in your lungs. Also, that style uses a very high volume of vapor (solvents) to a low ratio of active ingredients --versus the newer "salt" nicotine vapes that are low vapor volume, extremely high active ingredient content. The solvents are usually different ratios of VG and PG, which aren't 1/2 as bad for your lungs as tar and metal. Honestly, if I took a huge rip of the nicotine vape after smoking a bowl, it felt like it cleaned the nasty resin taste out of my lungs immediately. Idk if a solvent could even work that way to remove a superficial coating of tar from your lungs, but that's how it felt and tasted.