Rule of thumb: Whenever a clearance has an instruction to proceed from point X to point Y, where the path between those points is at the controller's discretion (such as vectors), and you have declared a comm failure by squawking 7600, you are to use your own judgement to proceed between those points by the most logical and reasonable path.
So if my clearance told me to expect vectors between two fixes and I now had a comm failure, I would just navigate my own way between them using whatever route keeps me clear of obstacles (probably direct), at or above the minimum charted altitude appropriate to the area (vectoring altitudes can be lower - you are expected to climb to the minimum published altitude following 7600 if it happens while on vectors).
On an arrival, if there is a comm failure on an arrival that involves radar vectors to final, you are expected to navigate yourself to the final approach course by the most reasonable and logical path, not lower than the charted altitude for the airspace you are in.
It's similar with the matter of timing; you are expected to start an approach at the last mutually confirmed or flight planned or expected time, so say if you arrive at a clearance limit early following a comm failure, you are expected to hold on your own as necessary so that you can proceed to the approach when you are supposed to.
The objective is that when the controller is notified of your comm failure when you squawked 7600, he/she will expect you to find your own way between any points that are discretionary at charted minimum altitudes, following a logical route and with timings that the controller can anticipate so the controller can route other traffic around you, confident you'll be able to avoid hill tops or tv towers on your own.
Much of the IFR rules structure is designed to ensure that an airplane with no communication capability can complete a trip maintaining its own obstacle clearance and proceeding in such a way that the controllers can anticipate where you are going with reasonable certainty. The key thing is to maintain a safe altitude and avoid going off in directions that make no sense to the controller as you find your way around.