Actually done by scaling, not transform. Procedure is: - set pivot to cursor (main keyboard period) - move 3d cursor to desired coordinate (e.g. another object that has the X coord you want for this vertex) - select vertex to be moved - hit 's' for scale - hit 'x' to constrain to x axis only - type 0 - hit enter the vertex will jump along the x axis, effectively copying the exact x coordinate (only) of the target object.