I just got my order from fstoplights. Not sure whether this is a good indicator, but they're not fitting into my vapor flask. The VF manufacturer suggest only using VTC series and LG HE2s. I got a pair of authentic VTC4s and LG HE2s, both of which fit perfectly in the flask. The VTC5's I got from fstop don't and actually seem identical to a set of VTC5s I found a while back on Amazon which also don't fit (way too snug to fit even with moderate force).
Based on this I'm inclined to suspect they're not authentic. I also have a couple really old authentic VTC5s I bought a year or so ago which DO fit in the flask, which only further leads me to believe these ones, unfortunately, are not legit.
I guess the quest for real VTC5's continues.