You may have noticed the Bulls haven't gone very deep into their bench lately, especially since the return of Tyrus Thomas from injury. It's not that Vinny Del Negro wouldn't like to...it's more that he has settled into a rotation and substitution pattern that he and the players are comfortable with. Hinrich, Thomas and Salmons can all play multiple positions, therefore even though Vinny is only using 9 players, it's really as though he is using 10 or 11. The Bulls have some versatile guys and that keeps the end of the roster off the floor.
You also may have noticed that the Bulls won 4 in a row after Kirk and Taj became starters about 10 days ago. I don't think there was any cause and effect to that. A lot of people make a big distinction between starting and coming off the bench, but, as the old adage goes "It's not whether you start the game, it's whether you finish the game." The Bulls have been getting off to good starts in games - even the 3 they have lost recently - and I think Tyrus will continue to come in off the bench because of the energy he brings to the 2nd unit.