A couple of comments. Please don't shoot the messenger - I'm just putting these out there
From the quick bit of research I did at an earlier stage in this thread, fitting aftermarket HID bulbs to non HID equipped vehicles is illegal so you may have insurance issues in case of an accident, or may fail an MoT in future. According to the DfT Information Sheet on Aftermarket HID Headlamps dated May 2010:
In the Department for Transport's (DfT) view it is not legal to sell or use after market HID lighting kits, for converting conventional Halogen headlamps to HID Xenon. If a customer wants to convert his vehicle to Xenon HID he must purchase completely new Xenon HID headlamps. The reason for this is that the existing lens and reflector are designed around a Halogen filament bulb, working to very precise tolerances. If one places a HID "burner" (bulb) in the headlamp, the beam pattern will not be correct, there will be glare in some places and not enough light in other places within the beam pattern.
And a bit further on:
Therefore a HID headlamp unit sold in the aftermarket should:
1. be type approved to ECE Regulation 98 as a component;
2.when fitted to the vehicle should enable ECE Regulation 48 to be complied with (although no government inspection will take place); and
3. comply with RVLR as far as "use" is concerned.
In practice this means:
1. The headlamp unit (outer lens, reflector, bulb) shall be type approved to ECE 98 and be "e-marked" to demonstrate this. That can only be done by the headlamp supplier - Hella, Valeo etc. - who must test the headlamp in an independent laboratory.
2. Once fitted to the vehicle it must have headlamp cleaning and self-levelling (which can be for the headlamp or can be in the vehicle suspension - some expensive estate cars have "self-levelling suspension" and that is adequate). Also the dipped beam must stay on with the main beam.
3. The headlamp must be maintained in good working order, kept clean, and aligned/adjusted correctly in the same way as any other headlamp
Good luck pursuing VW for free replacement, and hope it works. The challenge will be that, unless there is an actual fault in a specific headlamp unit, replacing the lights on any one vehicle is admitting that the H4 lights are not good enough. This will instantly spread through the media and they would potentially become liable to replace the lights on all T6's with H4's. VW sell about 200,ooo T6's per annum, so that means there are about 300,000 sold since launch. If 75% have H4's, that is 225,000 vans. If VW's replacement component cost is say EUR100 per van that would mean a liability of EUR 22.5m, plus fitting cost plus reputational damage.