Total failure on my part. The link I had sent is the wrong documentation page, and still points to a Blueprint node document. Please disregard 
Assuming you can get the actor rotation using GetActorRotation(), you can construct an FVector from it while only setting the value for X, 0.0f for the other two:
FVector xVector{GetActorRotation().Roll, 0.0f, 0.0f};
I believe this should be the equivalent of GetRotationXVector node, if I am understanding correctly.