The ‘x’ in SunTour Vx was widely understood as standing for ‘extra light’ (I do not know whether this was SunTour’s real intention). This was the SunTour V T Luxe after it had been on a strenuous diet, the steel holder for the cable clamp bolt has been replaced by an aluminium channel and the knuckles are slimmer and neater.
Despite the weight loss, this was an extremely affordable, strong and reliable derailleur - I still see many of these in use on the roads today.
This version of the Vx came out first, in 1976, as a mid length cage general purpose derailleur - but without a Quick Cage. It was initally just called a ‘Vx’. The next year, 1977, SunTour decided to develop the Vx as a family, and this version was renamed ‘Vx T’ but gradually faded away in favour of the, very similar, Vx S. The swooping pulley cage is rather elegantly formed from a single piece of aluminium, but is distinctly ‘un-SunTour’ in style. This prompts me to think that this model may have been created for a particular manufacturer. It is possible that someone like Schwinn may have demanded a derailleur without the Quick Cage for some spurious safety reason.