I hope that you find one. They are not plentiful. The Vincent design does not lend itself to large tanks. The fuel tank is pushed way forward on a Vincent. This means the forks and handlebars must have clearance. The oil cap needs to be taken care of which limits height. The tank can drape over the sides like a Hoske or Heinrich, but unlike a BMW, you cover up most of the useful and beautiful parts of the engine. Finally, the bottom of the Vincent fuel tank was highly engineered to prevent cracking. Many of the racing tanks used a stock tank bottom as a starting point. Almost all the racing tanks were steel not alloy, which I believe says something.
David