Rani, for once, and probably once only, you are incorrect on the info I know about.
VGN has not directly inherited any slots which were Nigeria Airways. BAA would not protect slots for anyone over the length of time since WT operated. BAA use ACL to manage the slot allocation on a use them or lose them basis, and as several years have passed since WT operated there is no way in the world that they could be linked to WT!!
If the slot remains unused due to its unsuitability to any other route that is a matter that is totally different