Originally Posted by Andy M.
I do not think it would be safe for three weeks.
I don't know what the pH has to be for it to be safe.
I didn't mean safe for three weeks. I meant put 'em in the fridge and let them marinate for a week and finish them by the time they had been in the fridge for a total of two weeks, giving only one week of eating time.
Did some googling. Looks like the pH that kills botulism is 4.6 or lower. But, there is a gotcha, "While most bacteria cannot survive at a low pH, some proteins such as in soy and beef have protective agents that allow them to grow at pH < 4.5." from http://edis.ifas.ufl.edu/fs10
So, I would recommend pickling the peppers, then adding the EVOO so it can absorb flavours. When ready to serve, soak the cheese and prosciutto in some of the EVOO and vinegar, and then stuff the peppers that will be eaten that day.
It would probably be a good idea to parblanch the peppers before pickling.