If in RNAV airspace then the aircraft is RNAV equipped and when cleared direct to xxx then their "normal navigation" will be to route from present position direct to that point and they will perform to the required RNP.
If in non-RNAV airspace or if the RNAV has failed then the radar controller should provide magnetic track to the waypoint when vectoring is complete and since they are by definition required to monitor the flight can provide additional guidance if required.
If procedural, then you will not be asking this question.
Spitoon,
You don't have to worry about Great Circle because aircraft following airways have always been following great circles because that is what radio signals always follow. Over a distance such as STU to DVR the difference between great circle and rhumb line is minimal at the mid point.
Regards,
DFC