I don't know anything about vert.x but did vert.x switch to MQTT v5.0?
IBM MQ only supports up to MQTT v3.1.1:https://www.ibm.com/docs/en/ibm-mq/9.3?topic=reference-mq-telemetry-transport-format-protocol
So, if there is a configuration or setup parameter that tells vert.x to use MQTT v3.1.1 protocol then change to to MQTT 3.1.1 or simply use the previous release of vert.x