Tower Supervisor described a near MVA separation loss when trying to comply with a IFR release time restriction; the supervisor noting he/she did not follow his/her own advise regarding allowing a release time to alter good decision making.

Narrative

[Airport was] departing and landing Runway XYR; landing Runway YX. Runway XYL [was] closed. [The] advertised runway in use was Runway XYR. A VFR aircraft; Aircraft X (a PA32); was departing Runway XYR. [I] took a hand off from the South RADAR Sector; Aircraft Y (a C421) IFR on visual approach to Runway YX descending to 3;000 FT. Aircraft X was issued a 160 degree heading climbing to 3;000 FT in accordance with facility procedures. There was a jet (a C560) with a release time that I was trying to comply with behind Aircraft X. The 160 degree heading placed Aircraft X 20 degrees off of the reciprocal inbound heading to Runway YX. Aircraft Y was on a dog leg to Runway YX. I became uncomfortable with Aircraft X and Aircraft Y on semi-converging courses. I issued traffic to Aircraft Y and then instructed Aircraft Y to descend to 2;500 FT abeam the carrot inside of a 3;000 FT MVA and turned Aircraft Y 10 degrees to the right to ensure separation. The Controller-In-Charge reminded me of the MVA in the area. Subsequent review of the TARP data revealed Aircraft Y was 1 mile east of the carrot turning northeast bound away and past the carrot when the aircraft began descending. Review also revealed that; had I let the aircraft alone or simply vectored Aircraft Y without descending Aircraft Y; the two aircraft would not have been a factor. I was attempting to ensure separation between the two aircraft. I preach to my own employees not to let hitting a release time lead them into a potential problem and I did not follow my own advice. The two aircraft were not a separation issue; but I put myself in a potential trap situation with the MVA. Recommendation: coordinate a different heading for Aircraft X. Attempt to coordinate a new release time for the C560 if I couldn't make the time. Simply turn Aircraft Y without descending the aircraft ensuring lateral separation and not both lateral and vertical.