The Vega is not so similar to the folkboat family. I know there was a Swede who had a Vega with a junkrig. If you look through old JRA-newsletters before 2000, you may find it. I think the Vega should be good with a JR, but one issue has to be sorted out:
A new rudder has to be hung on the transom to stand up to a sloop JR. The original Vega has an inferior rudder tucked behind the "short, long" keel, where it provides too little turning moment to control the boat.
Arne
PS 5.8: No, it was an Allegro 30, not a Vega