I know there is a drive to ban plastic, but without any doubt the best way to protect a fuel tank when fitting over the oil filler neck is a cut down plastic drinks bottle, choose one with parallel sides, remove the top and bottom whilst also cutting to the desired length, slit vertically and join back together using electrical insulation tape making it a tight fit round the oil filler neck, IMHO a far superior method than using a smelly old sock, or even a new one. Once used you can tuck it away in your designated Vincent toolbox for future use.