I want the selected vertexs position to be aligned with other ones position that is in blue color. The level of the wished vertex location is in red color:
Turn on the MeaureIt tool and add a segment measure to the edge above the blue line to get it’s exact length (increase the measure to show 3 or 4 decimal places first).
Now add another one to the edge above the vertex that you want to move and use the vertex slide tool to slide it while watching the segment length. Stop when it matches the other segment length.
Press down the Shift key while moving the vertex for better accuracy.
Is the red line parallel to an axis (i.e., x or y)? If so, select the one that is in the correct position, and set the cursor to it. Next, with the cursor as the pivot point, select the other vert and scale to 0 (zero): press S Shift-x 0 (zero), and enter (or S Shift-y).
Edit: Was on break at work for the above. At home, I tried the same method except using the coordinates set to normal, and it worked as well with any orientation to the global axes. So just find the axis between the verts, and use Shift- to exclude that axis from the scaling operation.
Edit 2: Since videos are getting all the attention, I thought I would re-propose my solution which requires no add-ons nor extra geometry. It goes fast so feel free to pause to read the shortcuts used:
Until we get snapping with edge/vertex slide, this is always going to be an approximation. The way I personally would tackle it is with Miratools CStretch with just 2 points, select the two ‘end’ vertices, hit the CStretch tool, click in the view, rmb to end and there you go, a nice aligned section of points. Cstretch is extremely useful for this sort of thing.
I set the snaping element to “Vertex” and then during transform I hit X to move along the X axis of the custom orientation I previously created, and hold down CTRL to snap.
Why don’t you just set the transform to normal, set snapping to vertex and drag the vertex on the X (or y or z whatever it is) and snap to the other vertex you want?