AFAIK Wessinger Vs Vetter is still pretty much relevant guidance for judgements in Product Liability cases, particularly in matters of jurisdiction, and relationships of the original manufacturer to the consumer. I've found cases as recent as 2012 that refer to the judgement.
Re enabling VIM - it is currently not illegal for the owner of a vehicle to make such a modification (as you stated above) but the use of such a device (after modification) does contravene Regulation 109, and the Met Police in 2013 have already stated that such users (drivers) of such devices (e.g.: Google Glass) that displays "directly or by reflection any cinematographic apparatus" will be prosecuted. I doubt that a court would find otherwise.
With regards to modifications....... it's only a matter of time.....
The EU Roadworthiness Directive !
http://ec.europa.eu/transport/road_safety/topics/vehicles/inspection/index_en.htm
http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32014L0045&from=EN
Albeit still in draft form, but nevertheless contains the phrases:
"relevant vehicle approval legislation at the time of first registration"
"according to the date of first registration or use specified in requirements"
"unsafe modification.... which affects the road safety of the vehicle or... adverse effect on environment"
"appropriate measures to prevent adverse manipulation of, or tampering with, vehicle parts and components that could have a negative bearing on required safety and environmental characteristics of the vehicle"
"verification as to whether the relevant parts and components of that vehicle correspond to the required safety and environmental characteristics that were in force at the time of approval"
A few UK police forces have jumped the gun?, e.g.: and published:
"Big bore and sports exhaust systems are usually fitted to increase the sound emitted and this contravenes the Type Approval of the vehicle, which is an offence."
from:
http://www.norfolk.police.uk/safetyadvice/roadsafety/advicetoroadusers/howlegalisyourvehicle.aspx
and
https://www.durham.police.uk/Information-and-advice/Know-the-law/Pages/Vehicle-Modifications.aspx
(Although arguably the Police could cite the RTA 1988, Part 2, Construction and Use)
But the EC Whole Vehicle Type Approval is already active for LCV's up to 3.5 tonnes (April 2013) and has already affected us at work, with respect to modifying vans, to put items such as roof racks, beacons, steps, towbars, shelving etc.
Thankfully at present, the UK VCA has an amendment for such items, and we have had to put all our LCV's through the "N1 Enhancements Scheme" to prevent us presenting our modified vehicles for a second Type Approval.
http://www.dft.gov.uk/vca/vehicletype/type-approval-for-go.asp
http://www.dft.gov.uk/vca/additiona...for-goods-vehicles/n1-enhancements-scheme.pdf
I think it is only a matter of time, before modifications in the UK on passenger cars will be scrutinised under regulatory powers.