SunTour took the V T-Luxe and replaced the last bits of steel with aluminum pieces, and slimmed down some of the parts to a slimmer and neater package, the result was the Vx. The Vx was introduced in 1977.

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The first V was made in about 1970. 

Disraeligears shows a Vx first type as 1976 only, in medium 59mm cage length without the quickcage or the pillar to stop the cage rotating too far back.  

The Vx second type is shown by Disraeligears in 1977 and by Ron Kitching in 1984 (unchanged?) in three lengths:

RD-2200 Vx    (short 47mm no quickcage)   200g 26T cog / 24T wrap
RD-2500 Vx-S (med 59mm quickcage)  213g.  Suganoya.net 26T/27T. RonKit'84 26T/28T wrap. Suntour say 30T cog /28T wrap on long 29mm hanger.
RD-2400 Vx-GT (long quickcage)  Suganoya.net and RonKit'84 give 34T/34T.

Early Vx derailleurs have a fixed top pivot stop in place of the B-screw; the B-screw had eclipsed the fixed stop by 1980.