Easy way to do this...just activate snaps (at the 2.5d setting). Then you can move in a given axis, but determine how far to move by snap to a remote point. Unfortunately, the help has always seemed a bit unclear to me. I've set up a simple example, imagine my spline in the first picture. Say I want to move one vertex straight down until it matches the vertical dimension of the vertex to the left.
In pic 1 you can see that I set the move tool to only move vertically.
In pic 2 is the only fiddley part....position the mouse so you get a lit cross in the center of the circle but WITHOUT activating the horizontal move axis. The screen shot doesn't show my mouse, but it was directly over the circle with the cross in it at the center of the move tool axes.
In pic 3, you can see that now...I can move that vertex only vertically, but the mouse if free to wander horizonally also...and when I land on the "guide" vertix, it SNAPs, and the result is what you are looking for, a vertex moved in one axis and lines up perfectly (vertically) with a vertex in a different horizontal location.
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