Applying a post rotation to a matrix which already has position data will cause the position data to be rotated as well. So if the position is 1 meter forward and you post rotate the matrix 90 degrees around the Y-axis, the new position will be 1 meter to the right. It seems like you just want to set the rotation of the matrix to the intersense rotation. In that case use
setQuat instead.
Code:
mainViewMtx.setQuat(inertiaCube.getQuat())
The Vicon plugin receives data in a separate thread, so it does not block the main graphics thread. So getting sensor data from the plugin will return cached data from the last update.