"Make arch" addon v 1.0.0

Install add-on to Blender.
Don't forget to enable it, after install!

After that new button "Make arch" in Edit mode should appear.
Addon wouldn't work, if you are not in edit mode. Also addon wouldn't had any effect if there is less than 2 vertices in edit mode are selected (and affect only selected vertices).

Center of your circle segment is 3D Cursor position. You should place 3D Cursor to proper place, before using the add-on (any way, Undo works for addon pretty well).

After those step, you'll get vertices aligned to circle segment. The arc radius could be adjusted in add-ons menu.
Also you can align your vertices to fixed axes (X,Y,Z) or to your current screen rotation, instead of aligning to single point.