How to align two vertices on two planes? I have 3 vertices here which I try to align, the bigger User Orth view looks down along the 2 which already are aligned over each other.
The upper left view shows a sideview and the arrow represents the axis at which the big view is looking down along.

I'm trying to get the selected vertex inline with the 2 behind it, the problem is that I need to restrain it on 2 different axis which are in different orientation spaces:
The main orientation is the big view's camera, 30deg down along that edge.
But my problem is that I also need to prevent the vertex from changing it's global z because I want the top surface to stay flat.

What I've managed so far was using vertex snapping with the big view's transform space set to view and locking view z to snap it inline.
That'd work fine if it wouldn't still mess with the global z slightly anyways.

So is there any way to align that vertex with the ones behind it but keep that top surface flat?
While in the bigger User Ortho view, select the vertex you want to move, the one that cannot be moved in the Z axis. Once selected hit G, then Shift+Z (this allows the vertex to move in the X and Y axes, but not the Z axis). Then, holding Shift to limit movement speed and increase accuracy, visually line up the vertices. While not mathematically exact, you can zoom up quite close to ensure they are aligned to your liking.