The -800 and -900 is not limited by Vmcl or Vs, the approach speeds are limited by aft body clearence. A -700 and an -800 at the same weight would have different Vrefs in order to provide more body clearence for the longer -800.
The NEI problem can be avoided/reduced by landing with less fuel. This time a year I aim to land with less than 4 tons and usually the frost comes right off after uplifting warmer fuel. A big "gotcha" is when landing in rain with cold soaked wings and low temperatures since the hold over time after de-icing is very short.
In the summer it's rarely a problem, but i've seen frost on the wings in temperatures above 20 C after landing.