So I have been using this macro:
/cast [stance:3] Ambush
/cast [stance:2] Ambush
/cast [stance:1] Ambush
/cast [stance:0] Backstab
It's worked perfectly this entire time, utilizing Ambush when I am in SD, and Vanish, also utilizing Backstab when not. However tonight (whether this is just a one time thing is yet to be seen) it kept back-stabbing first when I popped SD, then Ambushed. I only noticed it because the CP generation was less than usual and my Shadow in recount had Backstab damage instead of the normal Ambush/Evis.
So did something change? Or is this just some issue I might be having tonight? Maybe there's a better macro out there? Anyway, thanks for the input guys.