Hello Ben,
If you have the Vicon pose estimate coming in over your ROS network, you can access it directly in Simulink through the use of the Subscribe block.
If the information coming in over ROS from Vicon needs processing to translate it into a usable pose estimate, that can still be accomplished. If the processing is done out of MATLAB, you can simply have the code re-publish on a different ROS topic, and have Simulink listening for that. If the processing is done in MATLAB, you can do that too, but there are better alternatives. The most efficient would probably be to add the processing to the Simulink model as native blocks. An alternative would be to put your MATLAB code that does the translation in as a MATLAB Function Block.
-Cam
