Part of me says no, the Lady forbids this entryway because it doesn't utilize portals. Now if a pact binder is already being ridden by a vestige then they can enter Sigil, but the actually summoning doesn't work.
Part of me says most of the time, the Lady would allow it. Though perhaps a vestige like Tenebrous isn't allowed into Sigil, just in case Orcus finds a way to recapture the lost divinity he possessed (mind you, IIRC this idea that Orcus isn't divine is a weird shift from Planescape to 3.5e).
I do think there should be something off about vestiges in Sigil, but that's based around the theory that the Lady herself is a vestige able to act to an extent on the physical plane.