2x 18350's fit just fine in the Vamo in 18650 mode (aka both battery tubes)...get good Efest 18350's and all is well (one battery tube you can use 1 single 18350, add the 2nd for 18650 or 2x 18350 mode)
You can
buy battery spacers that should allow you to be able to use an 18500/18490 in 18650 mode
Remember to use them as mated pairs if you use 2x 18350's
Oh you are trying to do a 2x 18490 stacked setup (aka 3x battery extensions)?
Just
buy 2 extra's then double stack some 18650's...but your Vamo will look like a spear!
http://www.rustbeltvapors.com/Vamo_V2_Extended_Tube_p/vamotube.htm
Crap outta stock
Do a shout out and see if someone will sell you their 510 stripped out or dead Vamo
Oh wait...DUH...I have 2x Vamo's...just tested it with 2x 18500's...works fine with 3x battery tubes
OK, 3x battery tubes
Next to a MVP2 for size comparison
Using 18500 flat top Efest, first off they wouldn't make contact so you have to use nipple top batts, 2nd they were VERY close to the end, using 18500 nipple top might not give you enough room to tighten the end cap down, I used a piece of tin foil to get the unit to operate for the test only (not advisable!) Nipple top 18490's should work I would guess
YMMV...good luck