A defining vocabulary for the simplified Wikipedia and/or Wiktionary is a very major project. It has special copyright issues since the very few (three or four) publishers of w:idiom dictionary and translation dictionary products from/to English are very protective of their word lists and usage-of-word lists.
The regular Wikipedia has some vocabulary concerns of its own, but unlike a dictionary or reduced-English dictionary, it has no need for a defining vocabulary, since words are not being defined and readers are assumed to be familiar with English, within limits. The core glossary of the various projects must be compatible, e.g. words that appear as commands on the user interface, like 'editing' or 'diff' or 'log in', must be used in ways consistent with their usage in the actual text, which is simple to ensure if these words already are defined in the vocabulary of the simpler projects.
As Wikipedia, Wiktionary, translations and simplified versions come to serve larger and larger numbers of people, the acceptable vocabulary must shrink, as the hundred millionth user, five hundred millionth user, one billionth user, and three billionth user will have less and less grasp of written language, and will require more aids to understand these articles. New articles will be subject to scrutiny and editing by those who may imperfectly understand the content, and may insert poorly written passages, leading to restrictions on who can edit what page, and ultimately destroying the wiki spirit.
The sooner we think about this, the better, as we can hold it off as long as possible. So:
Please enter your suggestions for how to build a common minimal vocabulary in which to write entries for the Wiktionary and the simplified Wikipedia here. Implications of applying that vocabulary to restrict or guide editing of the Wikipedia proper should also be discussed here (rather than extending the discussion in the file on vocabulary, which is more devoted to ethics and extent of words and usages). If you think Wikipedia needs no such vocabulary, and can comfortably ignore the Wiktionary and simplified Wikipedia in such respects, then please say so:
See also reading level