Before I saw your reply, I was about to make a post questioning your mechanic's ability to ever get this engine running right for you given what we have heard up to now. But it does sound like he's trying, so maybe you're prepared to give him another shot.
Vacuum pump failure is theoretically possible in such a way that oil gets into the vacuum system, but remember (and remind your mechanic) the two reasons why his idea of oil being sucked into the intake from vacuum lines is impossible:
1) This is a diesel engine and therefore there is no vacuum in the intake manifold, as a gas engine would have. In fact, since it's a turbocharged engine, there is often *positive* pressure. How would oil be sucked in to get burned in the cylinders?
2) Due to #1 above, unlike a gas engine, on this engine there is also no connection between the vacuum system and the intake manifold! So, suppose the vacuum pump does implode and force oil into the vacuum system... how is that oil supposed to make its way into the intake?