Thanks for your reply.
Just manually, placing them in the FS2 library, then activating them through wxlauncher.
As I said, I've played through the main campaign using the 2014 MVP's without a glitch, and Vassago's Dirge seemed to have worked fine (although didn't start playing it, I quit through the first intro video).
Crossing the Styx's own main menu, and in-game HUD loaded fine in the tutorial mission, then the first mission briefing worked fine as well – until, as I said, clicking on weapon selection, when it crashed.
But the fact that I haven't been able to launch the game since the crash is most peculiar – tried disabling everything, and still no success.