Depends on the mood of the vacuumer (sure to be bad if she has stepped on lego or been had her requests to tidy lego away ignored), the prevalence of lego pieces, the attitude of the owners of the lego towards picking it up themselves, the location of the lego and what is already in the bag.
However, I do regularly ask DS1 (8) and DS2 (6) to pick up their Lego because I'm vacuuming. They learned that I was serious, the first time I did it. Everything went in the bin. I figure it's their loss.

Mind you, I do have the biggest collection of Lego you've ever seen. I mean, the boys have the biggest collection...