There are several Storage Backends in Vault, and only some of them supports HA, like Consul. However, if a backend doesn't support HA it doesn't mean that it can't be used at all.
So, if you need to run multiple Vault istance, each one independent from the other ones, you should be able to use S3 as a Storage Backend. But if you need HA you need to use Consul, or any other backend that support HA.
Hope this help