You can have multiple databases. Each database would need dedicated nodes. With a 6 node cluster:
DB1 on node1, node2, node3
DB2 on node4, node5, node6
In order to maintain high availability, each database would require at least 3 nodes for a K-Safety level of 1. If the databases loses a node with K-level 1, the database will run normally.
The way Vertica is designed is intended for a single database instance. Vertica falls under the MPP (Massively Parallel Processing) category. Multiple databases would be competing for resources on a cluster. The parallel design enables the distribution of storage and workload across the nodes. The best design is to logically create your schemas like you would databases.