Generally, on the morning of maturity, the CDs will be removed from your account and cash for face value plus accrued interest will be credited. Then you reinvest as you like.
However, if you signed up for auto-renewal when you purchased they will auto renew. I know Fidelity offers this on some of the CDs, not sure of Vanguard. However, again, for auto-renew, you would have had to specifically indicated that when you purchased if it was a new issue. If you are unsure or don't remember, pick up the phone and call customer service and ask. If you did not specify auto renew - kick back, there's nothing to do.