[What can I buy with daily commendations?]

To answer briefly:

 

- No, there are no other vendors than the ones you have seen.

 

- Yes, you can obtain Mods and Enhancements from doing the dailies. The Heroic on Illum gives you an Enhancement, and the Heroic on Belsavis gives you a Mod.

 

- So you can obtain all of your Armoring/Mod/Enhancement/Hilt/Barrels/Ear/Implants from doing the Illum and Belsavis dailies.

[Jedi Knight chapter4 Doomsday puzzle?]

Focus on Vitiate, Interrupt his long cast called Force Crush or something similar, use all your cooldowns, and you can beat him.