Is robot_pose_ekf required the tf in odom(wheel) to VO odom ?

[vishalleo1]

When we use the robot_pose_ekf for fuse the robot odom(wheel) and VIO odom, In this case the odom(wheel) is start from the initial pose of robot and this initial pose works as reference frame to the wheel odometry, same in the VO odometry for reference frame. Now here robot_pose_ekf is use the relative difference in the both odometry, then here we required to provide the transformation between the odom(wheel) frame to VO frame? because the monument of robot is same in the environment but the odom(wheel) reference frame and VO reference frame, this two frame are not perfectly align to each other then the change is x-axis of robot is different as per both reference frame, so how the robot_pose_ekf handle this case, and how robot_pose_ekf fuse odometry and provide the combined odometry of the robot. As per the robot_pose_ekf package we get the combined odometry of robot with respect to the robot center point.
thank you in advance for feedback!