After selecting the dropper, actually click on the preview window in Vegas, right on the video itself. You can seek to a different part of the vid that might be better for that selection. (IE. Be outside on Array when choosing your high adjustment color, rather in the tunnel below or w/e)
Dunno if I'm understanding the issue, but when you're at the 'Render As' dialogue, make sure 'render loop region only' is unchecked.
If you've got yours as above & have render loop region only on, it will do that.