If it is problems in Spain you are worried about and you go where I do, which is the Bay of Rosas on the North Mediterranean Coast, I think it is extremely unlikely that you will have any problems with any officialdom there at all.
It has been a long time since I have been asked by any official to present any form of documentation and I have kept various boats there on and off over the last 20yrs. Tempting fate saying this I suppose and no doubt next time Iím out there will be loads of them giving me a hard time asking for this and that.
The Rib I have down there at the moment was trucked down a couple of years ago and I did give the haulage contractor a copy of the vat invoice just in case but they said they didnít need it. I also sent down on the same vehicle half a dozen packing cases with no paperwork at all apart from a contents list.
I had planed to truck my current boat from Spain to Scotland but have put this on hold for a bit. I have not anticipated any problems with UK customs when coming back into the UK.
There are however, stories of people falling fowl of officials in the south of Spain but I am not aware of these difficulties relating to VAT documentation. I think there are one or two other members who occasionally come on this forum who live and or work in Spain and they may be able to advice of any potential pitfalls in their area. There should be some threads on the subject if you search around for them.
Having said this I do always keep onboard laminated copies of documentation such as SSR, Radio Licences, Insurance Certificates and Certificates of Incompetence etc but no vat invoice of purchase. Incidentally this year my broker sent insurance certificate through in English, Spanish, French and Italian. I didnít ask them but they just did it.
Hope this helps a bit.