Marvin
Your electrician is correct. Wiring for lighting is designed for just that, and should have fuses or circuit breakers to match. Overloading by using heavy current devices such as vacuum cleaners could result in damage to the wiring and in a worst case scenario could lead to a fire.
If power points have been added to this wiring by enthusiastic amateurs they may not have effective earthing. Some years ago there was no requirement for light fittings to be earthed, so an earth connection may not have been readily available. If there is no earth it is quite possible that in the event of a fault developing in an appliance connected to the circuit the metal casing of that appliance could become live. If the circuit was earthed then the fuse would blow or circuit breaker trip cutting off the power.
These days there are additional protections available in the form of RCDs, which are highly desirable but not always fitted to older installations, very unlikely to be found on a lighting circuit.