Does the 63º H reduction procedure only apply to interior ledge

The 63 degree is used when the ceiling height is below the window head height. I would have to say that you cannot use interior windows through vestibules since there are multiple layers of glazing. This is what on considers a borrowed light scenario which was not accepted using the glazing factor calculation, which was a formula. I would also say that using a weighted average between the two glazing areas would also not be acceptable. The area behind the vestibule would have to day lighted from other glazings. If it is not that large, consider the area behind the vestibule as a non-daylight zone. Combine this area into your calculations. If the intent and design of the space was to receive daylight through the vestibule, then use simulation.