Using a (vlax-curve-getfirstderiv obj (vlax-curve-getparamatpoint you can return the angle at that point so will be tangent to an arc just convert to 90 direction and like Lee check for readability.
So should be able to change lee's code pretty easy.
(defun alg-ang (obj pnt)
(angle '(0. 0. 0.)
(vlax-curve-getfirstderiv
obj
(vlax-curve-getparamatpoint
obj
pnt
)
)
)
)