Safe? Not completely.
How unsafe? Or how safe? Or how dangerous?
It really REALLY depends.
Buy good batteries, and the right batteries. Use the right charger. CHECK that you're getting the right voltage off the charger.
Use only either protected Li-Ion batteries, or safe chemistry batteries. Check the discharge rating (c-rating) of batteries before you buy them. If it's not sufficient for your vaping, do not use it.
NEVER mix pairs of batteries. (as this makes it much easier to accidentally put two batteries with different charge levels together.) NEVER NEVER put two batteries of different charge levels together.
Never buy or use non-rechargeable batteries. Don't even keep non-rechargeable batteries that look anything like your batteries around. (My wife used to use non-rechargeable cr123a batteries in her camera. I upgraded the camera to rechargables, threw the old ones away. Because you do NOT want to accidentally charge/use a non-rechargable battery.)
Check your batteries occasionally. MAke sure they're taking a charge right and holding a charge right.
Do not drop your batteries.
If a battery starts acting weird (for example: getting hotter than usual; lasting dramatically more/less time than normal; cutting in/out; looking worn; smelling burnt) THROW IT AWAY.
If you learn about batteries, and are reasonably careful, vaping (and stacking) is safe. But there are a lot of people in the community who seem to take the "it's safe, anything goes" attitude. And those people are NOT safe.
Best,
Ande