The inlet tube is about atmospheric pressure, the crankcase would have a positive pressure if it wasn't adequately vented. Right now the crankcase pressure is slightly positive due to blow-by, which is pushing the oil vapor through the tube. Neither the crankcase nor the inlet ever have a significant vacuum, that's not how diesel engines work.

BTW, the stock oil separator is there for a reason. Do a search, we've been through this about 100 times already. The oil separator collects the majority of the oil from the vent and drain it back to the crankcase, your setup is putting all that oil through the turbo and intake manifold, which is a very bad idea. Apparently your stock setup wasn't working correctly, or your blow-by is excessive. In either case, keep the stock equipment, you will never be able to sell the car as-is. Good luck.