Is it a requirement that 3D vector features must have the same projection as the stereo imagery being used in order to display consistently in the stereo window?
The features and the stereo images can have different spatial reference. What is important here is the target coordinate system of the map document.
Here are 2 scenarios on how the map spatial reference is being set if user is not pre-setting the map spatial reference:
1) It is new map document and oriented images load first
In this case, because there is no map spatial reference found, hence the target map spatial reference will be similar to the spatial reference found in the first image. If the feature has different projection, the map spatial reference should be changed to be the same as feature’s spatial reference.
2) It is new map document and feature classes load first
Since there is no pre-set map spatial reference, the target map spatial reference will be same as the feature’s spatial reference. If the spatial reference of the stereo images is different from the target map spatial reference, the stereo images will re-projected.
It is necessary to pre-set the coordinate system of the map document to that feature’s spatial reference you wanting to work with before you add or import the imagery. Or you can always change the coordinate system of the map in Data Frame Properties Window, Coordinate System Tab after you add the imagery.