Replacing 10 AAA NiMH 1.2A batteries with 6, 9v Lithium 1.2A batteries
The vacuum has a "don't catch on fire" device. Likely a PTC resetable fuse. The higher voltage means a higher current which means a tripped fuse. Or a heat sensor for the same reason.

In either case, lower the voltage or replace with nimh batteries. They are cheap.