Not a good idea. Sorry. Vinyl shrinks over time. This will cause movement which will crask or pop up tile. The vinyl glue will eventually fail as well. This will cause movement and ruin your tile job. Sheet vinyl can mask cracks in the concrete. Cracks create movement and will crack tile. Vinyl, whether cleaned or sanded, is not a good surface to adhere thinset. The vinyl really should be removed.

Hmmm.... I've read a lot of reasons as to why not tile over vinyl, but that's the first about shrinking over time. How much time would that be? The vinyl is 18 years, is in excellent shape and the glue is solid.

Vinyl is plastic. It will change shape with heat, cold and over time. It may not be noticeable to look at, but it will move. The glue is also flexible. Not good for tile.

Since you've been doing some research, you already know it's not a good idea, but you want to do it anyway to save yourself some work. It's a gamble; and not one I would take. Tile is expensive.

If you want expert opinion, check out the John Bridge forum. If you are really just looking for someone to tell you your installation over vinyl will be okay, you will probably eventually find a hack to give you a thumbs up. There is a reason you are having a difficult time finding someone to tell you to go ahead and install tile over vinyl. If you insist on doing it anyway, clean the vinyl or sand it first, as you like. It will make little difference. Surface prep is the least of your worries on a bad substrate.

The tile will not properly adhere to the vinyl. If you don't want to remove it, you will have to put down plywood over it.

The correct method would be to remove the old flooring however my father-in-law applied self stick tiles 10 years ago over his sheet vinyl in his kitchen and it's still there today. No problems. He did coat the old floor with some sort of watery stuff (sold at the tile place) first.

I also did the same in my half bath. It was actually recommeded there since the original tile may have contained asbestos.

As a temporary fix I even installed cheapie self stick tiles ($1 each) over concrete. Most every one stayed in place for years (guess it turned out not to be temporary) until we had a minor flood. Even then most stayed stuck. I had extras and just replaced the ones that came up.

Note some tiles stick better than others. I have found the thinner ones stick better on uneven floors.
Of course, multiple layers means more work if you ever eed to remove everything and start over.

We used porcelain tile over vinyl tile that had been down solid 35+ years. We didn't want to take it up because of asbestos in it. Wire lathing was applied over the tile and in about 3 years or so we now have hairline cracks in some of the tile. I would take it up. Not worth the risk. I would also assume some of the older tile has asbestos in it, and take the proper precautions in removing it.

Do not install plywood over vinyl adhered to concrete...very wrong advice.

Yes, that plywood would make a bad sandwich. Have you considered floating a quality laminate in a tile pattern over the vinyl? Mannington NatureForms has one that is a high pressure. Pretty nice.