If you want a five spell rotation with 15 and 24 second cooldowns, with mixed resets go with a click cast sequence versus a castsequence. This is what I use for grinding, server response lag will get that devouring plague if your even half way button mashing.

/stopmacro [channeling]
/castsequence [nocombat, harm, nodead] reset=combat Shadow Word: Pain, Devouring Plague
/castsequence [combat, harm, nodead] reset=combat/6 Mind Blast, Mind Flay, Mind Flay, Mind Flay

i would guess yours would be something akin to this. Basically DP is a 75% chance opener, with a castsequence either you need to overwrite your vt or you need to put another string of mb, mf mf mf.

Personally i find vt a waste in the rotation, it eats almost two mind flays.

/stopmacro [channeling]
/castsequence [nocombat, harm, nodead] reset=combat Shadow Word: Pain, Devouring Plague
/castsequence reset=combat/target/24 Vampiric Touch, Mind Blast, Mind flay, Mind flay, Mind flay, Mind blast, Mind Blast, Mind flay, Mind flay, Mind flay