Can I make a Vault bigger?

Yes, while the Vault is not canceled and its last payment did not fail. Open the Vault (click Dashboard in the top bar, then the Vaults tab, then Open on the Vault) and, under Change this Vault, pick a bigger size under How many panoramas?. You pay the difference between the two sizes for the time that is left, in proportion (and VAT, while VAT is charged). You can add years in the same payment, under How much longer?, unless the Vault renews by itself: stop the renewing first. Your change shows what each part costs and one total, and the new size applies as soon as the payment is confirmed.

A Vault is never made smaller: the sizes below its own are greyed out. Make a Vault longer or bigger has an example.