If the fluid level is really low, that could cause a slight flare. Make sure you check the level with the tranny hot, otherwise 5-10mm below the normal level is ok.

New vacuum pump should not make any difference. The cable to the tranny only controls the speed/RPM of the shift point, not the firmness.

When was the last time you changed the fluid & filter, and did you also drain the torque converter? This should be done at least every 30kmi, if not sooner. Using synthetic fluid is even better.