I was at first tempted to say it's a heating vent... but then I saw the second picture with the tube pulled out. It really looks like galvanized plumbing pipe. Though now that I think about it, the house is too old to be having galvanized pipe. If it's galvanized duct pipe or stove pipe, however, that would cement the idea that it's a heater vent. Very unusual to have a tiny vent. That's a very modern concept (interestingly used for retrofitting old houses).
But NOW I look closer at the first picture, I see a radiator valve right next to the stairs. So that kind of precludes that the house has radiant heat, not air. BUT. I don't see a radiator! Is there a tiny radiator under the stairs? And that's a tiny heating vent to let the heat up from the radiator? Ah ha, I'd bet that's what it is.
To answer your question simply, no it really does not look like a central vac inlet. It's a vent grille of some kind, there's no way to attach or insert a vacuum hose.
Some old houses that were built with no expense spared, have all sorts of neat oddball features, some we would never think of today. My previous house had a radiator in the linen closet to keep your towels warm, and in the detached garage to keep your cars warm (those were defunct, sadly).