Dear RC: A vocabulary of under two hundred words would be enough to ask for food, clothing, a place to sleep, directions and a few other things. I doubt very much that it would suffice to be able to understand much of the newspaper, and very little of the laws which must be obeyed.
Somebody should write a computer program capable to testing size of vocabulary with reasonable rapidity. Then a reasonably accurate estimate of a person's vocabulary could be obtained fairly quickly, and administered to large numbers of people, an results analyzed to give good approximations of vocabulary size in different groups.