If you have found new, unprotected 18650 batteries - congratulations! skip to the next step

WARNING: the protection circuitry is there for a reason. Shorting an unprotected battery will result in sparks and overheating.

Why do we need unprotected batteries:

the protection circuitry, among other things, disconnects the battery if it's being shorted (i.e. the discharge current is over a certain threshold).

as a result, when the vacuum tries to pull the current necessary to run the quite powerful motor, the battery protection circuitry shuts down preventing normal operation

To compensate for the missing battery protection, the vacuum has airflow cooling for the batteries, as well as a thermal sensor (which is what I think the red wire glued between the batteries is, see photo) which should shut it down in case of overheating.

Also, the protection circuitry adds an extra 2mm to the battery tallness, which means that protected batteries don't fit in the battery assembly housing

Follow any tutorial on Youtube about "how to remove 18650 protection". For example this one