Can't help with the problem Pete. but I have a Vito B and love it, I have taken some terrific shots with it in fact it was the first classic 35mm camera I ever bought, for the grand sum of £29, about 15 years ago and still going strong, just a suggestion that it could be something to do with the film advance, to test this turn the wheel in the center top of tje frame, tis is turned by the film sprocket and is the wheel that sets the shutter, if you turn it untill the shutter sets and the film advance lever locks and the shutter does not fire then you know it is something to do with the film advance, hope this helps you find the problem
Richard