The blocked vacuum cleaner example is interesting...
Normally you would expect any blockage to increase the load on the motor causing it to draw more current. Indeed some makes have a pressure relief valve that opens if the pressure difference is too high (indicating a blockage) I think my Dyson has this.
However I suppose there might be another possibility... A fan is basically an aerodynamic device. If the airflow is blocked this might stall the fan blades causing the suction to reduce and consequently the load on the motor reduces automatically. I think some F1 racing cars stall their wings to reduce lift and drag (or was that only allowed in 2011?). I'm not sure this actually happens in a vacuum cleaner but I suppose it might.