WotLK Pets and aggro

[acousticscars]

Hi,

I've decided to level up a warlock alt and at the moment I've reached lvl 27 and I've all 3 pets available (imp, voidwalker and succubus).
I'm demo/destro(8/11) and I'm facing big problems with aggro of my pets.

Here're my talents:
Demo--> demonic embrace, fel vitality, fel synergy
Destro--> bane, aftermath, cataclysm and 1 talent on improved shadow bolt.

I usually summon the voidwalker but the 1st thing I've noticed is that it can't keep aggro, neither on 2 mobs even if I use torment. Furthermore sometimes it resets, I mean stops to attack the target and come back to me.
Then health funnel works, but it doesn't heal him very much instead or at least not the same amount of hp I've to sacrifice, almost nothing and most of all even if it should be a tank pet its hp drops very fast even against 2 mobs of the same or lower level.
And I've 3 talents on fel vitality but the stamina/hp hasn't increased, at all.. Same fel synergy even if I do a quite high dps with my gear made with tailoring and 3 heirlooms(ring + 2 trinkets).

This turns off very much to level up this alt because I can't understand why I get all the aggro BEFORE to cast the spell.
For instance if I send the voidwalker and cast AFTER immolate I get the aggro before to finish to cast it. So I'm forced to use fear, or pop sacrifice if available and torment doesn't work neither on low level mobs (50 % miss).
Suffering same thing. Sometimes works, sometimes not and with a 2 mins cd is not that good.

Am I doing something wrong, or is voidwalker a bit broken ?

Ty in advance for suggestions