Some (not all) MVP's seem to have issues with certain 510-threaded juice attachments. I have a MVP-1 which will accept ego threaded and some 510-threaded devices without an issue, but would not make electrical connection with my Ikenvape cartomizers in my IBTanked cartotanks.
Since I also use a couple of Provari's, it wasn't a big deal for me to be limited to Kanger Protanks and iClear 30 clearomizers on the MVP. Between those two, I prefer the iClear 30. Larger juice capacity, dual top coils, 16 wicks, adjustable mouthpiece, better flavor and vapor production.