Am i allowed to bunker an ethereal slann inside TG as i thought that ethereal chars arent allowed to join non ethereal units and vice versa
I think the ethereal issue is a hold over from a prior edition. Currently, the only limitation (as per page 68) is that a non-ethereal character cannot join an ethereal unit but the reverse is freely allowed. A common vamp tactic to exploit this is to fill the front rank of a unit (usually zombies) with wraith heroes and then march straight into anything that doesn't have magical attacks. Also, HSoC does not make a slaan ethereal, it merely makes them immune to non-magical attacks.
I have a VC buddy who tried that tactic. He doesn't quite have the grasp of how it works though. And if the unit is flanked, I don't think you can makeway/combat reform since it would take models out of combat.
The wraiths are allowed to make way if they are not already in contact. Consequently, the best way to deal with such a unit is by a combo charge over the front with just about anything and on the flank with a real hammer unit that can beat the stuffing out of zombies. The characters will thus already be in combat and cannot "make way" . It also prevents the unit from reforming and moving the characters (other than to accept a challenge) since the unit is engaged over more than one side. The offset is that vamp players have considerable leeway with 5 wraiths in the unit as only 2 will be in the front rank upon the initial charge (the other three spots are occupied by command models). Thus, they can make way to either the flank or front as they wish to avoid anything with a magic weapon and to prevent the strongest non-magical attacks from hurting the unit. Consequently, having a scar vet with a magic weapon in the mix always helps and the added benefit is that he is higher initiative and can drop a wraith or two to make room for the unit to strike later at rank and file. Even a TG champ with a magic weapon (VoFF works well) can do great things as they hit on 3s and wound on 2s vs wraiths.