Up until now it been a basic rule anything made before 1960 is MOT exempt, but the new MOT rules as to MOT or not are going to be self governed for vehicles over 40 years old. This is when you go to tax a historic vehicle the DVLA will ask you the owner if the vehicle has been modified or not. If you say no then you will not need an MOT. The DVLA are not the instigators of these new rules, its the Department of Transport (DOT). The DVLA are only asking the question for their road fund application. If you get caught lying I dont know what the consequences will be, but one part who will be interested is you insurance company. Until now if its pre 1960 made they have no argument. After May 22nd if you have an accident your insurance company is going to question if your Vincent should have had an MOT as it was you who decided. They will wriggle, get the bike inspected and what if they see disc brakes on a bike that never had them fitted when new. They will probably say disc brakes are not a small modification of the original drum system (2 leading shoe is though). They will say its a completely different braking system so is a serious modification and so should have had an MOT and so your insurance is invalid.
There is a guide on the new MOT rules which the FBHVC published in their magazine which gives some modification info, but brakes are not mentioned. The document is as clear as mud but worth reading.