as i understand it:
if flying to a waypoint with a level restiction and then being a radar vector, it becomes impossible to meet the initial Vertical clearance as you will be crossing 'Abeam' the point and not 'over' it, so a new ATC vertical clearance must be issued. eg fly hdg xxx cross abm xyz at fl210.
if 2 or more heading changes have been issued and the cross abm clearance was issued with the first heading clearance, then no need to reissue the vertical part of the clearance as it still applies. ie it is still possible to cross abm xyz.
if the heading change is a momentary one and a clearance direct to the original waypoint is reissued then the vertical part of the clearance should also be reissued.
If the pilot thinks that he will eventually be re-cleared back on track then it will be smart to maintain the ROD incase the original clearance is reissued (dct to xyz cross xyz at fl210).
If Unable to comply just say so.