A snap on? It has been awhile but I thought these used the type with an expander screw something like a hollow wall fastner.
Try Bike Tools Etc, Loose Screws, and Harris Cyclery. maybe one of the many Vitus lovers will have a better idea.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk