Lighting spectrum plays a massive roll in the discussion as well.
Note that for years people have been growing bomb healthy happy plants under HID / fluorescent lighting. LED comes along and Bam - all of a sudden this VPD unicorn pops up as attempt to correct all of the shortcomings of the lighting spectrum being delivered to the plants.
It is the most unnatural way to grow a plant that I can imagine.
Purple Stems, mag deficiencies, overly compact internode spacings, odd leaf textures - I just can't dig it