Have the material stick out of the vise by a distance slightly larger than the radius around the edge.
1) Do the milling you mention, face the top and put the radius around the profile. This might mean you have to do the outer profile part way down but make it a few thou oversize. Do the triangle hole right through and put on the top radius.
2) Face and tap the ends on the large semicircle and do the hole that passes through the triangle.
3) Make a simple fixture that locates the part with a triangular boss in the triangle and a step the tapped ends rest against.
Clamp the part to this fixture with a strap across the semicircle and finish the radius around the rim of the triangular hole. Have the program stop and not put a clamp in the triangular hole just resting on the radius and take off the other clamp. Now finish the part.
I think I remembered everything .
An alternate way is to drill and tap the holes and the hole that passes through first then bolt the part to a block held in the vise. It should be possible to do everything from this block just by flipping it in the vise.