The only time you'll have multiple FV hosts configured for the same site is if you are running a Windchill cluster. In that case, you need to create an FV host for the physical hostname of each cluster node. What this does is create a separate set of mount points for each cluster node.
If one of the nodes experiences an issue where it can't access the vault directory, it will mark it's mount as invalid. But, since each node has it's own set of mounts, that means that the other nodes that can still access the vault directory will be unaffected by the issue. Otherwise, if you just had a single host defined using the cluster alias, then all nodes would share one set of mounts. If one of the nodes lost it's connection to the vault directory and marked the mount as invalid, then that would prevent all of the nodes on the system from accessing the vault.