To get Vorban you must capture all the monsters in Omega Ruins. You can find him in the arena conquest section after you've captured all the monsters. Use armor break and hasteaga to defeat him more quickly. He only has about 600,000 HP, so if you leveled up enough he'll drop the friend spheres in no time. Neslug is in the original monster conquest. To get him capture 1 of every monster in every area. I think this includes inside sin.