Tactical vectoring needn't be as specific as HD's answer - it could be as simple as turning an aircraft 20-30 degrees off track for 15-20 miles in order to give continuous climb.
There is no hard an fast answer to what tactical vectoring is - with area, approach and ATSOCAS to take into consideration, it could be any number of things.
You might even 'tactically vector' one aircraft in order to give some benefit to another... Any radar vector applied by an ATCO which helps them resolve a situation is a tactical vector.