I doubt there is enough fabric to miter the corners without adding a facing for the placket seam allowances. That would be a careful application to prevent bulkiness. Unless the person is one who wears suits everyday, I don't know that I would wrestle with trying to do it.

You may not have enough to re-miter, but it isn't necessary if you're taking that much off. Just sew straight and mimic the overlapped look. It doesn't matter if the vent is quite small. Once you sew the lining back on inside, no one is likely to notice if you do a neat job and reposition the buttons correctly.

It depends on size and the cut. Taking 2" off the sleeves means usualy take in the sleeve too otherwise you might end up w/much wider sleeve than before you started. So taking in the sleeve generaly leaves enough fabric to finish it the same way as it was done in the first place. I would recomend to put in new interfacing too.
There is couple of ways it can be done but if you want profesional job, specially on expensive suits, I'd take it to the tailor shop.
I worked in custom tailor shop in Boston and altered many suit. I had customer, every time he got new suit, he had it fitted to his body, re-cut the collar, shoulders etc. Kind of nuts.
I charge 15 dollars for shortening jacket sleeves. Where do you live?

czechchick2 - I would pay the $15 to not have to do it and I know how. LOL

Well, I'm in Florida. I'm not sure if it's worth it to pay for shipping both ways plus the alterations. You should be able to find some little old lady that sews. If you do it yourself, be carefull w/ the seam ripper :)

Thanks to all who commented.The garments are done & returned.I think I was just looking for someone to say it would be OK not to miter. The man that owned coats likely could have had them altered where he bought them.They had price tags ,$200.No doubt he was looking to save a few bucks.This was not my first sleeve shortening job & I have done the mitering but never on sleeves that needed 2 inches off.LOL I'm just hoping he measured correctly ! We all know how that goes. Iris