I want to edit the list of software repositories manually, but as this is the first Slackware-based distro I've seriously tried to use, I don't know where it would be. I'm looking in /etc, but that's only an uneducated guess.

If you mean the list of mirrors for slackpkg, the list is in /etc/slackpkg/mirrors. Keep in mind that slackpkg can only handle one repository, so uncomment only one in that list.

I've wanted to switch from the Debian family of distros to a Slackware distro (Vector, because I don't care to put in the effort to learn Slackware), but I've hesitated because of the differences in package management. (That was not what drove me away from Debian.) In the above, I should have said that so far, I can't find any of the package management files to manually edit. I'm trying again in a virtual machine.

I looked in the file manager for the folder /etc/slackpkg you specified, but there seems to be no such folder in /etc. All I can find relating to packages is /etc/slapt-get. (I understand slapt-get is unofficial, and have the impression Slackware experts scoff at it; but Vector offers it, and I intend to use it.) /etc/slapt-get contains two files: slapt-getrc and slapt-getrc.default. Reading the contents of slapt-getrc, I guess those are my repositories, because they appear to be the same websites listed in slapt-get. So is there any reason I can't edit this file with a text editor?

If the websites listed in slapt-getrc are my repositories, I'm surprised by the number of repositories--it's enormous compared to what I'm used to. Every Debian-based distro I'm familiar with has 10-20 repositories (activated or not). This file lists nearly 150, although only three or four are activated.