I have an anisotropic material model written in the UMAT. during derivation of the equations, it needs the fiber unit vector at reference state. Do I need to input that in the UMAT? Can I leave it there and in ABAQUS CAE, I define the orientation in the direction of the fiber?
It depends on the material model and the whole simulation. You can, for example, implement it directly in UMAT. The procedure was described in this famous article "Wilson, W., et al. "Stresses in the local collagen network of articular cartilage: a poroviscoelastic fibril-reinforced finite element study." Journal of Biomechanics 37.3 (2004): 357-366".