An excellent question!
The 2100' restriction is for noise abatement minima. The RVA chart allows for descent to 1700' within the final approach area, but if vectoring that's not much use (as we can't give you that level until it's too late!). Personally, if I'm vectoring for the NDB I'll generally give you "turn left heading 280, cleared NDB approach 24" and you can do whatever suits you regarding descent. Many pilots will descend the extra 200' straight away, then commence final descent at 6.1d as per the plate.
Where the 2300' figure comes from, I honestly don't know.