From the codex:
Teleport
"Teleporting is an effect that moves a creature directly from one zone to another. The Teleported creature bypasses all zones in between its starting and ending zone, thus bypassing walls and other objects along the way. It cannot be hindered by enemy creatures when Teleporting. Everything that is attached to the Teleported creature, except conjurations, including all conditions, damage, enchantments, equipment, and mana, always moves with the Teleported creature to its new zone. Any attached conjurations do not move with the creature when it Teleports, and are destroyed. In this way, a creature can effectively escape from conjurations like Tanglevine. A creature may Teleport into the same zone it started in (effectively Teleporting zero zones) to escape from attached conjurations. "
To answer your question, yes a wraith can use both of his move actions to do his teleport twice. He could teleport and then move out of the zone, into zone c, since the teleport action prevents him from being hindered as per the ability (It is not hindered when it enters the zone with an enemy creature).
The other big thing with teleport movement like Blue Gremlin, Grey Wraith, and Divine Intervention is that they do not require LOS to the zone they are teleporting to since it is part of the move action and move actions do not require LoS (You can move into a zone that LoS is being blocked by a [mwcard=MW1W01]Fog Bank[/mwcard] for example.)
