The best way is to use mod.ini files. You can use -mod directly but you'd have to set it up every time you swap campaigns. Mod.ini files let you set up once and then forget it.
This is my Transcend mod.ini file. All You have to do is make a new text file called mod.ini in your transcend folder called mod.ini and cut and paste this into it.
[launcher]
infotext = Only a few short months after the end of the Second Shivan Incursion, amongst the chaos of Capellan refugees and ever-heightening levels of piracy, mysterious things have begun happening on the edge of GTVA space. Unexplained disappearances, strange rumours... You are Lieutenant Sunder Marcel, recently transferred to the 103rd Shadow Guardians. Your squadron has been charged with uncovering the cause of these disturbances, to venture out into the unknown and return with an answer. But something's strange about this case. The clues have no meaning, and reality is beginning to slip... Everyone has a limit...;
website = http://www.game-warden.com/transcend/;
forum = http://www.game-warden.com/forum/forumdisplay.php?f=28;
[multimod]
secondarylist = mediavps;
It's only the last two lines that deal with your actual problem but you might as well add the rest. Once the file is in place you simply select Transcend as your chosen mod in the launcher and you'll notice the command line changes to -mod Transcend,mediavps
For more info check my FAQ's guide on how to install mods.