I would say your own scan is giving you all the information on the negative. The difference in the costco print is almost certainly down to some combination of their own settings plus post processing, which could include any combination of brightness adjustment, contrast adjustment, colour balance adjustment, noise reduction, sharpening, etc. The differences could be due to the basic costco scanner settings or their post processing. The costco print has also been scanned twice - once by costco when scanning the negative, and once by you when scanning the print.
The vignetting is still there in the costco print, just less noticeable because of the brightness and contrast adjustments.
When doing your own scanning, you would usually expect to have to adjust the scanner settings plus do post processing to get the result as you want it. As long as you've got all the information the negative contains, which I suspect you have. You just now need to get your scan into a photo editor of your choice and adjust brightness, contrast, colour as you see fit, plus noise reduction and sharpening to taste.
I would also say the original doesn't look like exceptionally good quality (a little soft, for example) and the costco print looks pretty rubbish too, so I wouldn't expect great things from this negative. But I wouldn't blame your scanner.