hi ,
whilst the cases are appart why not fit the spring,which ensures that the bevel remains in position in the cases, and holds it in the correct position in relation to the cam plate. If your cases were how ever still to gether I would tell you to foreget it, as the spring has no effect on the gear change at all , it was one off thoes nice little design features of the vincent.
I once knew a chap in the club who did a complete engine drop to change that spring in an attempt to improve his gear change. What a waste of time, it did how ever keep him happy.
regards stumpy.