Nancy Grace -- I'm NOT Counting on Casey Anthony's Vote!

Nancy Grace doesn't think there's a chance in hell Casey Anthony will vote for her on "Dancing with the Stars" -- but she DOES have some advice for the Tot Mom ... confess and go to jail.

Only one problem with Nancy's advice -- it's the whole Constitution thing, which prohibits double jeopardy.

Nancy says she's a horrible dancer -- and clearly not a big fan of the 5th Amendment.