I'm not sure about the first game, but I imagine the iMuse system presents some frustration in dealing with the second game. With PC games, it's also easy to get hung up on the exact best way to get to the music (in the game versus accessing the actual audio data in some manner).
Technically, you don't directly log VGM files with DOSBox, even VB's mod of it. You instead log files to the DRO format, and the version VB created just adjusts the DRO logging a bit to fix some bugs/make converting them to VGM files easier.
The key combination CTRL-ALT-F7 will begin the logging of a DRO file to the directory where DOSBox stores recordings (the "capture" folder), and hitting it again will stop it (and thus mean that the next time you hit it, it will start logging to a new DRO). You can change this in the emulator's keymapper (CTRL-F1), although you obviously shouldn't change it to anything you're going to be using in the games you plan to play.