Question about missing piece of siding

Prepare yourself: I fear this is one of those truly clueless questions, but I just know you guys will know what to do, and I really don't.
I was walking around my house doing a "get ready for winter" visual inspection and noticed there is a piece of siding (vinyl, if that matters) missing where the top row meets the corner of the house. It's on the porch where it is not exposed to the weather, but obviously it shouldn't just be left that way.
It's a gap of about 3 x 3 inches. It looks like the last piece used (the top piece) fell a few inches short of meeting the corner.
I know nothing about siding. Can I buy a little piece like this? Or do I buy a regular piece and cut to fit? (Already looked in the garage -- no leftover pieces.) It's the front of an outside corner, where the porch wraps around (if that matters).
Where the piece is missing, I can see board (looks like particle board of some kind). If I can't get a piece of siding up there right away, what should I do to seal this off? Tarpaper (ick)? Paint? Again, it's in a protected area where it doesn't get hit by rain or anything, but it is the front side of the porch (would be nice if the fix looks good).
Thanks in advance,
Jo Ann
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snipped-for-privacy@gmail.com wrote:

Even though the International Residential (building) Code says it's not required, there should be a weather barrier - house wrap or building paper - behind the vinyl siding. That is an issue in itself. If you know the manufacturer of your siding - or supplier or builder, get the name of the manufacturer and style and color, then order a piece. Also check on line for installation instructions. TB
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It sounds like a piece of siding simply slid over. See if the siding will slide over to cover the gap. I've had this happen before.
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Ronald, thank you for suggesting this! That's exactly what had happened. Love it when the fix is simple :-)
Jo Ann
Ronald' wrote:

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