I use the following for both demon attack and demon summoning since I'm demonology spec and I'm always summoning a felguard. Mod alt tells the demon to follow to effectively stop attacking and mod ctrl applies a health funnel. Otherwise the nice thing about this one is it will summon a replacement demon in combat if it goes away or dies, using fel domination for faster summoning if your in combat

/a [target=focus,help]; [target=focus,noexists];party1
/petattack [nomod:alt/ctrl]
/petfollow [mod:alt]
/cast [nopet,combat][target=playerpet,dead,combat] Fel Domination
/cast [nopet][target=playerpet,dead] summon felguard; [mod:ctrl] Health Funnel

if your not demonology spec (which sounds like the case since your using voidwalker not felguards) you can make it the following and it should do what you want as well as sending your demon in to attack

/a [target=focus,help]; [target=focus,noexists];party1
/petattack [nomod:alt/ctrl]
/petfollow [mod:alt]
/cast [nopet][target=playerpet,dead] summon voidwalker; [mod:ctrl] Health Funnel
/cast [nopet][target=playerpet,dead] summon imp