I am not seeing any attached photos, FYI. Would be easier to help if I could see the images.

But, if the piece of siding you're referring to is at the top of a "wall", usually the last horizontal piece is cut to height and then hole punched along the top edge. These holes then click into the top piece of trim, which is called "undersill trim". Sometimes the undersill trim is tucked inside a standard J-channel trim.

In any case the undersill trim and/or J-channel will be nailed to the wall, but the nails won't be accessible. To undo the piece of vinyl, try using a special vinyl siding removal tool. They are meant for "unbuckling" one siding piece from another, but also might be useful for unclipping from the undersill trim. Not 100% sure though because I've never had to do that.

If these aren't the trim pieces you're referring to, get some pictures posted and I'll try to help.

I forgot the photos? Ugh.

I do believe that the two trim pieces I was looking at are J-channel and undersill, per your photo, but if you can confirm that, I'd appreciate it. I'll do a bit more searching using those terms when I have a chance.

Thanks for the response sans problem description pictures.

Yes, based on your photos it looks like an undersill inside a J-Channel. Pretty standard.

I don't know if there is a specific tool for unbuckling the top piece of siding from the undersill trim, but you could try a vinyl siding removal tool like this:

It's meant for removing one piece of siding from another, but it might be helpful to get it out of the undersill trim, too. Fortunately the end of your undersill trim is accessible, so just start at an end and slide the removal tool in and work your way along, horizontally. It has a little hook on the end of the blade which could help pull the undersill trim outwards. As you do that, pull down on the piece of siding with your hand. Hopfully it will un-click.

Once you get it off, you'll need to cut the piece of siding down a bit to the correct height (don't cut it too short!), and then re-punch the top edge with a vinyl siding punch like this one:

If I remember correctly the punch is a little pricey at around $30 but it makes a specialty hole/impression that clicks into the undersill trim. Probably worth it to have the correct tool.

Let me know if that doesn't make sense.

Thank you for the detailed reply Nick, I really appreciate it. I'll look for the tools next time I'm at the home improvement store.