Aye the direct injection, two stage injectors and VNT turbo make them great on the veg, I think the problem boils down to the fact you don't need to really work the vag lump on a daily basis, plenty of low end grunt to get you by without much right footed input, which inevitably leads to grotty innards.
Best way with the water injection I've found (permanent basis), is buy just the "devils own" methanol injection nozzle, (15-30 quid) (one with a high gallons per hour rate), and couple it to a 12v water pump out of a caravan / motor-home shower (20-40 quid) , these usually have a modest delivery rate and operate between 60-100psi, which is perfect for the meth nozzle mentioned. Then either tap the nozzle into the inlet manifold or boost pipe connected to the manifold depending what you have to hand, & rig it up to a boost actuated switch (20-40 quid) that engages at about 1bar (max boost). Couple that lot with at least a 10 litre water tank in the boot, anything less and you have to refill it painfully often. You can also use the setup to fumigate all sorts of chemicals into the engine as well if needed. I used to run a 20% methanol / 80% water mix, found the meth improved the flame front propagation from the pilot injection and cleaned up the main burn, but anymore than 20% upset the lump at high speed and the high delivery rate needed for the steam cleaning effect of the water.
In a temporary pinch for a big steamy clear out, you can jubilee one of those plant mister nozzles onto your garden hose, take the inlet manifold hose off and rig it up so the engine is drawing the water mist from the hose in while running - then use a couple of bricks to wedge the accelerator down to around 2k - 2.5k RPM, and leave it for as long as you can afford to neighbor rage-wise. You want enough flow to produce some white smoke at the exhaust, and so that the engine is only fractionally "quenched", in that after a few seconds of drawing the water in the revs don't drop more than a hundred or two, and don't show signs of dropping further. Best to keep an eye on the proceedings as well.
Hah aye it was a good guess, I've already made all the veg mistakes over the years already mind, so it wasn't a guess born of any intelligence!