Virtual Dedicated Cloud architecture includes a causal cluster of 3 members. Aura deploys the 3 members in different availability zones.

An Availability Zone (AZ) is one or more discrete data centers with redundant power, networking, and connectivity in a cloud provider region.  This protects your database(s) from being affected by a failure in one data center (or AZ) of the underlying cloud infrastructure provider.

Therefore, as part of Virtual Dedicated Cloud tier you benefit from the resiliency of having the AuraDB cluster in multiple availability zones (multi-AZ).