ClusterXL monitors all interfaces (health-check) that were defined in cluster object's Topology.
If VLAN interfaces were defined, then, by design, ClusterXL monitors only lowest VLAN ID on that physical interface (starting in R75.47 / R77 and higher (including R80.x versions), ClusterXL monitors both the lowest and highest VLANs - refer to sk92826: ClusterXL VLAN monitoring).