Your Vapcell cannot be more recessed as my Samsung 21700…here is the answer to your question:
Remember when even the SkyRC MC3000 had contact issues with flat-tops too? Yes, this Vapcell S4 does make positive contact with the 21700, and all my other flat-tops. See why in the photos below:
SkyRC MC3000 left, Vapcell S4+ right:
The Samsung 21700 inside the bay still has plenty of room to spare, up to 75mm.
Photos transferred from my thread here: HERE