Demonic Pact functions properly if Life Tap glyph buff is active the first time Demonic Pact procs, but if Demonic Pact procs while already active, it won’t take into account the Life Tap glyph spell power when it overrides itself.
After one test with Vengeance of the Illidari, it appears to function properly with buffs other than the Life Tap glyph. But I don’t have enough outside sources of spell power to do proper tests to verify.