All development and patchnotes and betablogs of WotLK points to a change in mechanics regarding stacking - which means vampiric embrace most likely will not stack in WotLk.

It will instead proc a "mana-regen" buff on the 10 lowest mana raidmembers, called "Replenishment" - which gives you something like 0.5% of your manaregen being active, for a duration. It will no longer be scaling according to damage, but according to your own manaregenvalue ie spirit.

Hunters with hunting party and paladins will also get this ability.

In general, it seems that almost everything will become raidwide, however similar effects will simply not stack.

Unless specifically stated ofc.