Finally got to do the online check-in for my Vista 6/11 cruise. Logged in at exactly 11:00 Central time (Midnight in Miami), skipped all the details and headed straight to the check-in time. I was able to get slot A-04 at 10:00 - 10:30. Saved, exited the site and then returned to finish checking in. Printed the docs and the time slot stayed the same.
If I had done the complete check-in prior to getting the time slot, it may have pushed it later into the day. If I had waited until morning, it surely would have been later.
Just a tip for Carnival check-in.