A handy man will wind up typically setting the flashing portion ON TOP of the shingles on all 4 sides and then sealing it with plastic roofing cement.
This will keep it water-tight for only about one season, until expansion and contraction crack through the dried out roof cement and then rain will leak in.
Make sure the roofer that repairs it, describes how to get the flashings under the shingles properly.
Their is a good article on adding a vent and how to loosen the nails from the shingles on www.askthebuilder.com
Here is a specific article which shows a properly installed turbine vent. http://www.askthebuilder.com/442_Roo...ne_Vents.shtml
Note how the sides of the flashings and the top portion of the flashings are covered by the asphalt shingles.