First, the sauce:
Now, the rest:
Sauce first!
Put all the sauce ingredients except the sugar in a bowl/jar/large measuring cup and mix until the colour's even. Then taste it. Then add sugar until it tastes mildly sweet, which won't be less than 1T and probably won't be a lot more than 2T, but there's a lot of people in the world who like things sweeter than I do, so ymmv.
The salad part!
Boil the potatoes until they're squishy -- 20-30m. Drain the potatoes and put them in a large bowl until mostly cooled. Mix in vegetables and sauce. Fridge overnight for best flavour, and toss before serving to get the sauce off the bottom of the bowl.
The capers really make this one.