You've got me curious now ... in what respect are you all using the word "vocabulary"? I assumed it meant broadening one's understanding of words, which doesn't have anything to do with spelling or the 100 most common words in the language as far as I can see. I agree with PancakeGoddess that in the sense of expanding kids' vocabularies, reading aloud to your kids a bit "over their heads" is the best, most enjoyable way to go about it. Thanks to this approach, I have a 4-year-old who uses words like "pristine" and "collaborate."

For older kids, learning about Greek and Latin roots is an excellent way to expand one's understanding of the English language. We like Rummy Roots, and informal games while travelling ("let's think of all the words we can that come from the root "manus").
But maybe I have a different concept of the term vocabulary from the one most of you are using.
Miranda