You could always use redrob41's Republic Combat Suit skin (
http://www.lucasforums.com/showthread.php?t=197346) rather than the vanilla Republic Soldier. He's already given permission to use them in mods so long as you credit him.
All you'd need to do is create your armor based on the Combat Suit, and change the texture number to whatever you use for redrob41's skin. Way easier than messing with 2da's.
Edit: The model is "N_RepSold", and the best way to do it (if you're set on that model) would be to change the clothing model for whichever characters you want to be able to use the model. In "appearance.2da" you'd need to change the entries in "modelb" and "texb" to the model name above. Then you could create clothing based on the Republic Soldier model. Note that clothing appears differently depending on the entries in those columns - ie, if you put the same clothing on Mission (without changing her appearance.2da entries), it would appear as her default vest thing.