I've been vaping for little over a year now.
I vaped 50/50 exclusively and gradually lowered the nic levels, and other than occasional throat dryness, no major problems.
I recently tried 100% organic VG for the first time, it took a couple days but my god that thing wreaked havoc on my system, with endless clear yucky phlegm that just wouldn't stop.
It took me a few days to realize that the VG may be causing it, because I was coming off of a seasonal cold at the same time so I figured it was causing the phlegm.
However, after several days all cold symptoms went away completley but the phlegm stuck around like crazy.
I tried to think what could be causing it, and it dawned on me.
I now switched back to my old 50/50 and the phlegm is back to normal.
So I can say for me 100% VG is trouble. YMMV.