Say a party of adventures were in a flying vehicle about the size of a large rowboat and went through a portal to Sigil. Would the ship be allowed in?
Currently, only your DM can tell you, but there is nothing that speaks against it
This answer is for dnd-5e only.
There is precious little material written about Sigil's portals in any of the official D&D 5e books published so far. The PHB tells you that
This bustling planar metropolis holds countless portals to other planes and worlds. Sigil is a trader’s city. Goods, merchandise, and information come to it from across the planes. There is a brisk trade in information about the planes, in particular in the command words or items required for the operation of particular portals. These portal keys are highly sought after, and many travelers within the city are looking for a particular portal or a portal key to allow them to continue on their way.
And that's about it. The DMG adds that:
Portal keys of all kinds are bought and sold here.
With nothing else to go on, this is all up to your DM. There is no specific reason why a magical flying vehicle would not be able to pass into the city, if it is small enough to fit through whatever portal it is using. Portals come in many shapes and sizes. Find one that is large enough, and you are good.
Maybe the upcoming Spelljammer product will add more insight. Until then, how these portals work is entirely up to rulings.
From older editions, where there is more lore about Sigil, we know that gods or demon lords cannot enter the city, and that you cannot directly plane shift or teleport into it; it can only be entered or left via portal. But there is no ban on magic items entering the city. Considering this is a trader's paradise, including for exotic and magic items, it stands to reason that these magic items likewise must enter or leave the city via one or more of these gates.