On the vacs I have looked at over the years the most common fault is a wire breaking inside the cable - caused by repeated tugging - sometimes at the plug end and sometimes at the machine end - the fix is to work out which end and then shorten the cable by 6" or so. Wriggling the cable usually diagnoses which end.
You may indeed have a dodgy switch but my money would be on the cable itself.
Dysonmedic.com has words and pictures showing how to change the switch...
Good luck...
Peccavi
September 2010
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix vacuum cleaners.