Many folks have struggled to get the reference line on reference dimensions to be in the plane of the object (steel, vessel, etc.) only to have it come out as a diagonal.
Here's the secret, select the first reference point and second reference point in the plane that you want the reference line. This means that you will either select the center line or other line work in the plane.
So if you want to show a vertical line, you need to make your selections from the bottom to the top of the reference line.
For a horizontal line you would select from the nearest to farthest horizontal points on the line from the selected component.
Here is an example of using the View option to show the points. Notice the green line showing the reference from the component to the reference points (1&2).
This is how that is shown on an isometric.
It might be more beneficial to use the XXX SKEY so that the only text that will come up with the reference line is that which you enter in the Message field.