Is there a way to turn vex noise maps into vertex selections, be they solid on and off or fading from 0 to 1. ?

And then on to the next question of is it possible to turn those vertex selections into face selections (that is a dynamic way, not switching from point to face selection manually)

And is it possible to grow and shrink those selections dynamically?

A bit ambiguous. What kind of VEX noise maps? A shader applied to geometry? As a vex SOP applied to geometry? A noise image generated out of the compositor? Something else?

The real issue is wether you need to evaluate the noise in uv space or P position space. Either way, there should be a way to do what you want. The only real tough one is the shader applied to your geometry as shaders get evaluated at render time. In that case, you would have to use the mantra -u option to render a flat uv map out, apply the map to your geometry, then use the pic() function to fetch from COPs or the tex() function to pic up from disk in to a point SOP. Once the color info is on the points, then you can use an expression in the Group SOP, like $CR>0.5.

You can also write a vex function in SOPs to access the texture map from disk. Build a handy interface to grow and shrink the selection and build a group automatically.

the noise is comming from vex
i am currently making a vex sop to smooth the colour valuse of points so i can threshhold the valuse to grow and shrink selections.
and as a side noat do you know of any electric looking noise in vex or will i have to figger that one out my self? examples of what i am looking for

If the noise is coming from a vex SOP, then you can export a parameter, ampl that could be the noise amplitude. Follow this with an Attribute Create to generate the variable mapping so you can use the ampl variable directly in parameters. For the name of the variable, use ampl then put $ampl in the first value parameter to do this. You could then use a group SOP set to expression and use $AMPL > 0.8 to select points with a value greater than 0.8.

Electric looking noise is just thresholded sparse convolution noise, or snoise in vex. The turbulent noise VOP can use sparse convolution noise. It is a common technique to take lines or ramps (ramp VOP) or checkers and disturb the s parameter or Position P if volume based with turbulence utilizing snoise(). Then multiply this in to a constant color to get the desired effect.