nobelharvardsYou do not have to "install" vpks in a sense, in any way. What you have to do is dump them into the custom folder in your tf folder. The game will check that folder when it starts up, and do its thing with any mods you have installed.
Although it does not matter, if you wish, you can reset the file associations in Windows with certain file types.
I am using Windows 7. You will have to find your own way in XP, 8, 8.1 and 10.
In the start menu, a few bars below the control panel, there is "Default Programs". You can find the same thing by opening control panel and using the search bar on the top right to search "default" and it should find an option saying "Default Programs" or something similar.
Now you need to select "Set your default programs".
On the next screen, you need to find the application in question. In your case, it is notepad. Click on that under "programs" on the left.
On the bottom, will be 2 options; "Set this program as default" and "Choose defaults for this program". You need to select the one that says "Choose defaults for this program".
Find the .vpk extension. Untick it. The save button is in the bottom right corner. Unless you have other file extensions that you have "accidentally" associated with notepad, there should be no need to untick other file extensions.
Tell me how it goes. :)
I use Windows 8.1. I found what you were referring to but .vpk is not shown.