That's an interesting trail - it does start to look a bit more convincing when you see it all laid out like that!
One thing that springs to mind: when the Romans colonised England, quite often the Gaelic-speaking populace, pressed into becoming servants, were given common Roman/Christian names that were similar to their Gaelic names. Thus
Aine would be called
Anne, or
Aoife would be called
Eve, etc. Something similar might be happening here: Vedastus isn't a name that translates into English, but
Foster might have sounded similar enough that the English used it instead of Vedastus. The same might have happened between Vedastus and
Gaston in French.
WDYT?