Firstly, I apologise for the vinegar comment. I looked at the hand book and the reference is to putting vinegar in the windscreen washer water as it is detrimental to the washer pump. Culzean has clarified that point. As to deionised water, all I can say is I have had no problems with it. Now, water from the dehumidifier, my Wife used to use it in her iron until she did a decoke ( why modern irons need this was clearly illustrated) the crud that came out was quite amazing. We assume it is dirt in the air. We also have a filtered water system, as She has health problems and even that used in the iron produces copious amounts of crud.
Distilled water I can only surmise is even more expensive than deionised water. Where one can buy it I know not. I bought some deionised water from a motor factor and it was three times the price of good ol Tesco, so I am loathe to go to a factor, perhaps a good look around Tesco's might produce some. I hate the place but they do have a very good range of specialist food that my Wife requires, so trips there are infrequent and under duress. Moany ol git!!!!