The Sun Tour literature I've seen seem to indicate that the Vx will run with a 30t, anyway. They don't specify a long cage until 32t. FWIW, I run a V-luxe with a 28t with no troubles.
I also would guess that your Regina freewheel is standard thread. The one that came off my '73 Atala was, as are the three others I have in the freewheel bucket.
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