Just as it took a long time (partly due to the deliberate obfuscation of vested interests) to establish that tobacco was as dangerous as it is, so it's likely to take a long time (in the face of perhaps similar obfuscation from some of the same vested interests) to establish a sufficient body of evidence to show that vaping is harmful, and to determine the extent.
In the short term, where it's not clear how dangerous (if at all) vaping intrinsically is, there is a case to be argued that if it gets people off of tobacco, which we know to be lethal in a high proportion of cases, it's probably beneficial, as there aren't many things we do that aren't immediately lethal that are as likely to kill as smoking tobacco.
I recall hearing today (I think BBC's 'Analysis' podcast?) that 8 million people a year, worldwide, die of smoking related conditions; if vaping reduces that to, say, six-million with a further million vaping related deaths, that's a net benefit. It's not as good as nobody smoking or vaping, I'll grant you, but sometimes pragmatism has its place.
O.