Your billing depends on your sign-up date and has nothing to do with the date of the announcement.. If you signed up at the beginning of the month when they announce packages, that's when they're going to bill you. If you do choose renew for Vault 35, wait until the third week or so of the month to do it. That way you'll always be auto-billed later in the month of the announcement and will always have time to cancel between the announcement and the payment deadline.
What you said makes sense but apparently the next vault announcement is coming out later at least compared to the last time.