I would orient some instances by vertex normals, but i'd like to control orientation by vertex groups, to give z negative values to some instances, like in the pictures.is it possible? i've tried with Vertex Group Input but it gives me unexpected results.
You can achieve that effect by controlling the scale of the normals with the vertex weight.
Then, remap the weight (using Vertex Weight Input node) which is in range (0, 1) to (1, -1) using Map Range node with the Threshold (in Threshold node) = 0.5 i.e., vertices which have weight less than 0.5 will have normals with scale = 1 (+ve direction) and vertices which have weight equal or greater than 0.5 will have normals with scale = -1 (-ve direction).
You can very the Threshold as you like.