I want to assign same Y and Z coordinates to all my selected vertices. I think a way could be to drop them onto a vertical plane but I don’t know how to do that.
select the vertices and press s -> y -> 0 to scale them to 0 along the y-axis then do the same for the z-axis. This will give them all the same y- and z- coordinates
If your plane is coplanar with two of the global axes you can scale them to zero on the orthogonal axis. In your picture the plane seems to be parallel to XZ plane; in this case you would select all vertices then S->Y->0 (zero).
If your plane is not parallel to cardinal axes, you can use the Retopo Tool to drop vertices onto a surface.
Great!
I’ve solved with S>Y>0 and S>Z>0. But I’ll have a try with GeomTool too, it seems very powerful and it could be useful. Thank alot!
Another question for you guys.
What if I want to cut my mesh along a straight line and don’t make the trick of drop the resulting vertices to a plane to obtain a smooth edge?
There is a way to tell Blender to cut my mesh along a straight line and then automatically recalculate the faces so that the external vertices of the faces lie all on the same line?
up for an answer… 
I don’t know exactly what you are trying to do, but you could use the knife too (‘k’). Select the area where you want the cut, select the knife tool, click above it, and then click again under it and you should have a straight cut.
Thank you!