If I have a model, for instance UV unwrapped, how can I from a texture assign a vertex group. White = assign vertex, black = don't assign vertex, values in between = halfway assigned vertex based on color intensity. A grayscale image would probably be used.
You can try the Vertex Weight Edit modifier. For example, I can input the following texture as a Texture Mask to the modifier:
... that will then use its value, to control how the modifier assigns weight to each vertex. Tweak at least the Default Weight, threshold settings and fallof type to get the result you need:
Though not instantly updated if we edit the image, the result shows up immediately, and is usable by modifiers further down the stack:
Default Weight and Add Threshold should be of that weight which you'd like to have as the end result, Add Threshold should be slightly less.