Turns out, quite annoyingly, the package on Arch providing vlock, and an older version at that, is kbd, from the Keyboard Project (http://www.kbd-project.org/).

FYI to everyone on Arch, for a current version that makes this work, you need the AUR package vlock-original, and then invoke that as the executable name in the terminal. I already replace xscreensaver and got rid of it. Thanks for the tip!