Will x3 18650 batteries be enough for a nominal 12V vacuum cleaner?
Given that NiMH have a very stable voltage as they discharge, it’s quite likely that the motor runs directly from this supply, assuming that it will be more or less constant.  Running from a lower voltage will give less power.  This is more or less proportional to the square of the voltage so you’d expect 85% or normal power.  On the other hand, if the appliance uses a switch-mode supply to run the motor then you may see no degradation in power, although that would seem less likely because switch mode supplies cost money and 12v is a sensible voltage to use directly for a small motor.