@PPF. No, but what do you mean by fill in?

That vent is in an odd position too. I wonder if there was a wall that was removed?

Pull from a closet to get material? No way would I ever downgrade to vinyl instead of wood.

@PPF. Yes the prior owners took down a wall there. : )

If I used your recommendation, would you thread the planks or put a cross plank in? No contractor I have been in contact with wants anything to do with threading new planks in.

Oh my, can you have a little duct work done? That looks like a safety hazard. Did this pass code when they removed a wall but left the floor vent?

They need to be woven in. Need to find a better contractor -- it's a bit or work, but the only way to do it correctly.

I would recommend removing the ceramic tile and making both surfaces all one height with new wood flooring in the kitchen. A good wood flooring contractor can work in new planks. You may need to remove a few of the existing wood planks so that pattern looks correct. I would also move the vent, if possible, since it is in the middle of the floor it will continue to be a tripping hazard even if you change the flooring.

I am not sure why you would need to add a sub floor if you removed everything to add a vinyl floor? Is it because the sub floor is not smooth enough for vinyl? In that case yes you may need to add an underlayment.

I agree with doing the whole kitchen in wood, and your bedrooms if you have the money. If not just get new carpeting in the bedrooms, good quality carpeting with upgraded padding. In terms of the vent, I don't see why you would have to move it, your flooring team could just put in a new wood vent cover but inset it into the floor so it is flush and therefore not a tripping hazard.

I have had wood in two kitchens now and it has not been a problem. I would keep talking to contractors until you find one willing to weave the new wood floor into the existing floor. You will be pleased. A reliable A/C contractor should be able to relocate the duct to an area in a zone that is less conspicuous.

A good hardwood installer/ finisher can blend the woods since you are having the old redone anyway and IMO you could probably get away without that vent since it looks like it was in a separate space before.The way the tile is added in just does not work anyway go wood everywhere.

Looks to be 2 1/4" red oak...could be 3". Any wood professional should be able to lace in a new product, sand stain and finish and make it look like there was never any tile to begin with.