Viso2_ros covariance matrix problem and robot_locaization (ekf)

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hi, I want to use viso2_ros with monochromatic camera to determine the visual odometry and fuse it with the imu orientation. As viso2_ros does not publish any covariances so we used the pose and twist covariances given in stereo_odometry code. But still the robot_localization(ekf) is not fusing the visual odometry values with the IMU. I tried to run the ekf with only visual odometry sensor but the ekf can not read the messages given at the odom topic. my ekf_templete launch file is given here. I dnt know whether it is the covariance issue due to which ekf is not fusing the visual odometry :/

<!-- Launch file for ekf_localization_node -->
<launch>
<node pkg="robot_localization" type="ekf_localization_node" name="ekf_localization" clear_params="true">
  <param name="frequency" value="30"/>
  <param name="sensor_timeout" value="0.1"/>
  <param name="two_d_mode" value="true"/>
  <param name="map_frame" value="map"/>
  <param name="odom_frame" value="odom"/>
  <param name="base_link_frame" value="base_link"/>
  <param name="world_frame" value="odom"/>
  <param name="transform_time_offset" value="0.0"/>
  <param name="odom0" value="/mono_odometer/odometry"/>
  <param name="imu0" value="/imu_data"/>

  <rosparam param="odom0_config">[true, true, false, 
                                  false, false, false, 
                                  true, true, false, 
                                  false, false, false,
                                  false, false, false]</rosparam>

  <!--<rosparam param="imu0_config">[false, false, true,
                                 true, true, true,
                                 true, true, true,
                                 false, false, true,
                                 true, true, true]</rosparam>--> 

  <param name="odom0_differential" value="false"/>
  <param name="imu0_differential" value="false"/>
  <param name="odom0_relative" value="true"/>
  <param name="imu0_relative" value="true"/>
  <param name="imu0_remove_gravitational_acceleration" value="true"/>
  <param name="print_diagnostics" value="true"/>
</launch>