I am also working with a very similar setup. I have:
One Windows computer running Motive 1.10, streaming data using the vrpn broadcast.
One Ubuntu computer connected via LAN to the first, running ROS (including MAVROS and vrpn_client_ros).
I can launch vrpn_client_node with the Motive IP as an argument. This node will auto-detect trackers and send positions to /tf.
$ roslaunch vrpn_client_ros sample.launch server:=192.168.1.148
I then launch px4.launch in the mavros package. In my (unchanged) installation of the latest version, it launches the mavros_mocap_pose plugin. So no changes have to be done here.
$ roslaunch mavros px4.launch fcu_url:="udp://:email@example.com:14555" gcs_url:="udp://@192.168.4.2:14556
(i'm connected over wi-fi to the FCU. The second argument is the IP of my Ubuntu computer within the FCU's wifi. This enables QGroundControl to run without crashing mavros.)
Finally, to have the pose data sent to mavros, I use a relay node from the topic_tools:
$ rosrun topic_tools relay /vrpn_client_node/(tracker name)/pose /mavros/mocap/pose
(notice there is a space between the name of the two topics. Also, make sure the topics are of the same type!)
I can then check that the FCU receives the data by echoing the following topic:
$ rostopic echo /mavros/local_position/pose
I can also verify it in QGroundControl.
I hope this helps you, if not, I hope it helps the next guy