Looking at the schematic diagram for the vacuum lines (Book 2, Section 2-1, Page 13 of the Factory Service Manual), I can't see any place where oil could be getting in, since none of them connect to the crankcase in any way.
The only ones that are directly connected to the crankcase are the PCV hoses.
As for the diff smell, I don't know if its normal but when I changed the oil in my car's front diff, the smell was so bad it made me sick. Though if I remember correctly, mine smelled more like badly burnt cooking oil than rancid milk. I'd like to know what causes this too.
- John