I'm assuming this isn't possible for most SPCs. Here's the thing: from what I know, SPC files can have loop information compiled into them. This basically tell an application how long the song goes. Most SPCs don't include this, though, so playback applications can either "guess" the length or just play indefinitely until the user stops. The SPC plugin for Winamp, for example, had some special SPC settings where you could define the length of SPC files. By default, this was set to 3:00 minutes, and the way this worked was this: "if there is no length information baked into the SPC file, play the song for this long before jumping to the next song". From what I could find, VLC doesn't seem to have a setting like this (though to be fair, I didn't really look thoroughly). So it seems like VLC by default goes the "play forever" route, which means you probably have no other choice but to stop the song manually. If you need to, you can manually edit the output file in Audacity or something like that to add a fade-out to the end or shorten the song as needed.
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