There haven't been studies yet, as there haven't been concerning humans vaping diacetyl or AP. That's the point. No such studies exist- all that is available now is anecdotal evidence. Saying 5-6 million vapers haven't had issues yet is just as silly...
No, there is a HUGE difference. In the case of weight trends, it is easy to find people that gained weight, lost weight or stayed the same, after quitting smoking while vaping. You will find similar groups among people that quit smoking without vaping. Trying to carve out the impact of vaping, and VG in particular, would be a huge statistical undertaking. No one can do that anecdotally.
Contrasting that, there are simply no people known to ANY OF US reporting lung function impairment due to vaping. Those people do not exist, to any of our knowledge. If they did exist, and were known, you would be posting links as well as many other people. But you can't because they are unknown. That is a very easy statistical problem. It's somewhere around millions:zero. I have no doubt that if enough grant money were thrown at the problem, some outliers would be found. But it is incredibly unlikely that these people exist in large numbers but are hiding from us. It is even more unlikely, considering that the various vape-hating groups have not found them because those people would like nothing better than to trot out sick vapers to justify their demands to control vaping.
And once again, I have challenged everyone to post links to people reporting lung degradation. None have been posted. OTOH, I can google "stop smoking weight loss" and find all the examples I want, vaping or not. Two very different situations.