You are starting from the wrong end. EVERY Vaillant boiler fault find is started on the CH NOT HW side.
If the appilance doesn't attempt ignition, the procedure on a VCW goes as follows, this is ONLY a brief synopsis of the basics:
Does fan run?
Does pump run?
Does microswitch M2 make? (bottom of diverter pair)
Does FPS make?
Does PD valve operate?
Does appliance attempt ignition?
Is there 20vdc across solenoid?
I suggest you get an RGI with some experience on VC/VCW to try to repair. Try Vaillant themselves, I've heard they're quite handy on repairing these.
Carefree
October 2010