No doubt modern brakes may be a safety improvement but then I do believe that someone official with some technical background should cast an eye on the installation to approve the set. As mentioned here before there was damage to a Brampton by a powerful front brake so there is a safety aspect in the installation itself which is not a trivial mod.
As to passing the TÜV test , it seems they have got quite liberal in recent times, partly because there are some more organisations now certified for vehicle inspections. So when it comes to get disc brakes approved you will use a good strategy and find an inspector first and discuss pariculars with him before setting to work. In case you find him unwilling to discuss - just say thank you very much and find another one more open for your plans.
It is just a matter of insurance risks why you would want to get your mod approved and listed in the vehicle papers. In case of an accident the specialists from insurance companies will no doubt find out easily that a disc brake in a Vincent is not standard and without approval by TÜV or somesuch you are illegal and in consequence they will not pay out. A tls brake mod may look very standard so I guess the specialists would not get ideas. But then, that mod could be approved as well, even easier possibly.
Vic