Voting in the sandbox should be discouraged
I must respectfully disagree with Secespitus. The whole point of the Sandbox is for questions to change. In the perspective of an individual voter, they may change for the positive or for the negative. This means votes easily fall out of date.
Worse, there's no automatic way to tell a voter that a proposed question has been edited. So, unless you (the voter) plan on monitoring that question 24/7 to be sure your vote is still valid, the act of voting can be very misleading.
To that end, I believe that voting for proposed questions in the Sandbox is utterly meaningless and should be discouraged. Their potential to give the OP the false belief that a question is ready (or, worse, the false belief that they've been judged unworthy...) is far greater than whatever value they may have.
The better solution is to use comments.
If you want to downvote, don't, post a comment explaining why the proposed question isn't ready for prime-time.
If you want to upvote, don't, post a comment explaining your belief that the question is ready.
The beauty of the comments are:
The commentor can be informed of changes that may affect the ideas expressed in their comment.
The dates of the comments can be used by others to judge the relevance of the comment.