I had the same prob, I made a grease trap for the vaccum cleaner to suck all the water and oil out, then just wiped it with a rag.
just get a bucket with a lid, drill two holes, one for you vac hose, then get some 19mm or so poly pipe(or something) and stick that in the other hole.
Then suck!!
The bucket will trap all the oil and crap in the bottom and the vac bag will not be filled with crap. be sure to only stick the vac hose in the top half an inch or so and the other hose in a little further!