VintageSteelEU

Mystery solved. Sort of. I think.
So, one thing I noticed was that the hanger bolt protrudes quite a bit on the inside of the hanger. Yeah, you've got the nut to secure it, but comparing with Huret Success (which uses a nut on the inner side as well) I thought it's a bit strange to leave that much of the bolt. You'd like less of it to ensure good chain and cog clearance, right?
Following this line of thinking, I started looking at photos of other Vx (and Vx-S, Vx-T and Vx-GT). And the theory I came up with was that I have the version made to be used with with a dropout claw but I don't have the claw. Both frames use dropout hanger. Not having any spare hangers at the moment, I decided to get a thick washer and test the theory. Lo and behold: it's shifting all the way up to 7th cog and the cage is not getting in contact with the spokes. There's the problem of chain being too short for this derailleur and 7 cogs (Wipperman 708 is 110 links), but the problem of inner cage plate catching the spokes disappeared. The only adjustment I made, compared with previous tests, was tightening H adjustment screw a bit to move the pulley wheels underneath the smallest sprocket. This is where things get a bit hairy and I'm still processing the results. I had used Shimano derailleurs before with a claw and on frames with a hanger (the same derailleur) and it was always just the matter of adjusting the limit screws. But apparently it does matter when it comes to Suntour Vx. Just adjusting the limit screws is not enough.

For test number two, I pulled the pivot spring out from Vx-S and put it into SVX. After adjusting the limit screws it works perfectly OK. The chain is still too short, but other than that, it shifts merrily up and down, no problem hitting the spokes. Now, the difference between the two is the hanger pivot. SVX is slightly thicker (which probably doesn't matter) and its bolt is held by that "flathead" Suntour round nut, which ever so slightly protrudes on the inner side (I guess this is what matters), therefore acting as a spacer. There might be other subtle differences I'm not noticing (angle of knuckles perhaps?) that somehow move the parallelogram or cage tension pivot away from the dropout and spokes, the cage itself is of course different, but whatever it is, no sign of a possibility for it to get into the spokes. So the general conclusion is that with Suntour Vx it does matter whether it's the hanger or the claw version.

Since I had that SVX on the bicycle, I tested index shifting. It works, badly. As in: you might have a good run and all will shift correctly (although not as responsive as it should be) between smallest and say, 4th cog (didn't test above that because of the chain length) and back and the next time it's a no go. I'm guessing some careful adjustments might get index shifting more reliable, but I think adjustments only won't be enough. Perhaps a cable travel adjuster of some sort would be sufficient or maybe a set of shifters with a different pull ratio (SHimano 7400?) would do the trick. So that's something that requires further investigation.

grumpus

Just get some Suntour Power shifters and use them as nature intended.

JohnDThompson 

Derailleurs with mounting bolts that require a nut on the inside to secure the bolt tend to be those designed back in the 5- and 6-speed freewheel era. As more and more sprockets were piled onto the cluster, clearance became an issue. Huret addressed this in later production with a mounting bolt that use an expander wedge to secure the bolt instead of a nut:

VintageSteelEU

Got 3 sets, two with clamps, one set without a clamp (spare). Very nice indeed But I also do like indexing and I like tinkering and experimenting.