Methamphetamine is very, very stable. It stays as methamphetamine when it's vaporized, regardless of whether we're talking d-meth or l-meth.
> There doesn't seem to be debate that P2P processes create l-meth and d-meth, and that l-meth was less common in earlier versions.
Not quite. p2p done naively creates racemic meth, yes, but the modern methods, which have been used for years, purify that to enantiopure d-methamphetamine. They do this by bubbling d-tartaric acid through the racemic mixture, which separates the two enantiomers thus yielding the desired pure d-meth.
Furthermore, historically racemic meth was much more common. AFAIK the infamous "shake and bake" technique creates racemic meth. I do know that there is a pseudofed route that yields d-meth, but I think it's different than shake and bake. So don't quote me on this paragraph, but at a minimum we know that in the last several years, almost all meth seized in the US is highly pure/potent d-methamphetamine. There is less l-meth than ever.
> I just don't think the evidence is so weak that we can discard the potential that l-meth is involved.
On the contrary, the evidence is so weak that we literally can discard that l-meth is involved. First of all l-meth is better studied than you say. I would bet to approve the l-methamphetamine-based vics vapo inhaler they had to at least do animal models with large amounts of pure l-meth.
Second of all, and this is what blows the giant hole in your argument, there is no surge of l-meth. There is less l-meth than there has ever been in the history of methamphetamine.
Thirdly - this ties into the second point - you seem to thikn that getting racemic methamphetamine is something particular to the p2p method. It's not. As a general rule, synthesis of any compound yields the racemic version (if we're talking a compound with two enantiomers). This is definitely true for meth, where almost every method yields racemic meth. AFAIK there's a pseudoephedrine route that yields straight d-meth, but like I mentioned above the current state of the art is just to do a big p2p synth route and then separate out the d enantiomer specifically.