Couldn't you conjure up a V-block hidden inside a square block? Remove piece 1 with a square end mill. Use the new edges A and B as the bottom while you remove 2,3 and 4. Just have the original piece shorter than your vise's maximum opening, so you don't have to put the sharp corners against the jaws. Rather, turn the piece after the first cut (where you have the piece square in the vise with the longest side running parallel to the jaws) so you get to side mill 3 and 4 (probably have to turn the piece around in between cuts) and end mill 2 from above.
