Well the vamo v3 is shipped with the extension tube for 2 18350 and it comes with 2 18350 from most vendors too, so I don't think stacking them is dangerous or they wouldn't ship it that way new.
For one I'd like to have the ability to make it smaller with one 18350 and plus I've read of some really long vape times with 2 18350's
The extension tube is to use a single 18650 battery in the Vamo.
You don't think stacking batteries is dangerous? Every publication on battery safety cautions against stacking batteries. Google stacking batteries if you don't believe me. And you don't believe that a vendor would include an unsafe battery in a starter kit? Chinese vendors don't care about your safety; I've seen them include
un-protected ICR Li-ion batteries in starter kits -- batteries which should never ever be used in mods.
As stated by myself and anumber1, you'll see very little benefit to stacking two 18350 batteries in a regulated mod. You will not get more battery time, but less battery time than using a single 18650 battery.
Stacking Batteries <--- did you read this link?
You can lead a horse to water, but you can't make him drink it.