As a Corrpution Sorc where would I go to start my Ilum story quests? I can't seem to find the mission on Vaiken?

Say you buy/find a blue/purple crewskill mission. How does that work. When you use it is it a one time use or does it randomly show up as a mission available to do? I'm not really sure how this works. Any information would be appreciated.

Could someone point me in the direction of companion customization vendors for the Sith side?