It probably does matter where you touch it.
A common fault in vacuum cleaners is intermittent open circuit in the power cord and or plug. Constant tugging from overly vigorous vacuuming over time causes a conductor to part either inside the cable or in the plug.
Of all the vacuum cleaner faults I've fixed this failure type must represent perhaps 75% - mostly plug related.
Good luck...
Peccavi
January 2010