The Vamo is one of a few devices that are designed to allow stacking batteries. It doesn't matter if they're 18350 or 18650, the circuit board will recognize the higher voltage of the stacked pair and adjust its performance accordingly. With a stacked pair, the Vamo will stop firing and give a "Low V" error message once the voltage drops below 6.4v, just as it does when a single cell drops below 3.2v.
Stacking batteries increases the risk factor. Because of the increased risk, it is strongly discouraged on ECF.
It can be done in the Vamo, but only if you use the highest quality batteries available and religiously follow a strict protocol. The batteries that come with the Vamo kits are not suited for this purpose. That includes the Panasonic 18650B's that come with the 18650 kit. You will need to get AW IMR "high drain" cells from a reputable, stateside vendor like RTD Vapor.
Here is the protocol:
http://www.e-cigarette-forum.com/forum/vamo/452155-batteries-2.html#post10920157
Do not attempt to stack an 18650 and an 18350. A nasty accident is likely to occur.