I am using Efest IMR 18350 800 Mah (stacked) and Everfire ICR 18650 2400 Mah I also have some Everfire ICR 18350 1000 Mah batteries that I haven't tried yet. I ordered the Efest's with the Vamo after odrering it I looked around for extra battries I found the Everfire's on Amazon for next to nothing afer I used my rewards points

The stacked 18350's I had to lower the wattage to 6- 6.5 or it tasted slightly burnt. with a single 18650 I can vape at 7-7.5 watts with no issues. The 18650's are about the same diameter as the18350's so I have no battery rattle issues.
hmm...
I googled Everfire batteries and got nothing (same with Amazon search as well) Probably meant Trustfire or Ultrafire.
Anyway ICR batteries are not the recommended batteries for use with a Vamo.
The difference you are experiencing between stacked 18350's and a single 18650 is probably from being in AVG mode. (Make sure you are set to N02 for RMS mode)
At low wattage levels like that your juice shouldn't taste Burnt.
With my all day juice, I vape at 11-12W or 4.5-5V if I am using an unruly coil.
As to battery diameter, let's break down the numbers...
With an 18650, you have a battery that is roughly 18mm wide 65mm long hence the designation '18'65'0.