I played around with your code and was able to get it working.
Two notes. First you will need Super Duper Macro.
Second Note. You need to keep pushing the button , lol. Sometime it looks like the macro is stuck but if you keep pushing it (usually once or twice) the macro resets.
Aslo, if you did not take the talent power infusion you can ran remove the last line.
I find taking the talent Devine Insight yield more DPS, in min my limited testing, Especially when you have the opportunity to DOT many mobs.
Final note, since you only want to hit your DP when you have 3 orbs you will need to manage that spell manually. Also when you get your proc for a free mind spike , you will need to hit ALT + Macro Button. If that is not your style you can have a 3rd button .
/cast [noform] !Shadowform
/castsequence [mod:alt] Mind Spike; [nochanneling: Mind Flay] reset=target/combat/3 Vampiric Touch, Shadow Word: Pain,, Mind Flay, Mind Flay, Mind Flay, Vampiric Touch, Shadow Word: Pain, Mind Flay, Mind Flay, Mind Flay, Vampiric Touch, Shadow Word: Pain, Mind Flay, Mind Flay, Mind Flay, 13,
/castrandom mind blast
/castrandom power infusion