Hi,
both deployment modes should be possible. Jet is built on top of the Hazelcast IMDG, one can get the HazelcastInstance from JetInstance and pass it to Vertx Cluster Manager.
The benefit of having two clusters (one for Vertx and one for Jet) is the service isolation (e.g. lifecycles of Jet and Vertx clusters not coupled, no risk of noisy neighbours).
Vladimir