I bought a night vision monocular that calls for 5x AA batteries. Would two 18650s in series work/ be safe?
Yes/No.
Yes the voltage should be within the same range.
No, I cannot say it would be safe by default and lithium ion cells in series has a potential to be dangerous if not handled
properly. Lithium cells (both rechargeable and primary) possesses enough power that if shorted or discharged beyond safe current limits can result in fires and even explosions. When you put several in series you further add the risk of reverse charging into the mix and you cannot just put about 8.5 or so volts on a pair in series and not have problems, you need to charge EACH cell individually and have circuitry doing so that keeps them from being overheating while charging and not overcharging. Most series cell packs also contain temperature probes that the charger and the device the battery is used in monitor the battery temperature and shut the power off when it exceeds a certain level to prevent runaway heating (very dangerous).
If you are unwilling to put in the time to learn about all of this stuff...... don't do it is my advice not worth the risk.
If you can find a battery pack that has a built in protection and balance circuit and a charger to properly charge the battery that fits that would be easier.
A place to start to learn about making series cell lithium rechargeable packs I can think of would be the R/C community these people build packs for cars and drones etc and know about 100 times as much as I do...... likely a few also frequent here maybe they would chime in perhaps.