If you do not use 3 and 5 and the boiler has a visit from Vaillant they will not touch it until its wired as per instructions.
I agree Londonboy and have experienced the problem. Vaillant will do anything to get out of a warrant call. Fact.
I can't see the problem, you have to take a live and neutral to the programmer, and you have to take a switch wire to the boiler, so whats wrong with a 3core and do the job as per the manufacturers wiring guide.
Red. blue, yellow, 3,5,4 simple.