i i've set up the old blender 2.49b to import/export Nif models for New Vegas, i'm making a pretty large quest mod that i originally thought off but i wanna do some custom mesh but i'm very used to Blender 2.8 configurations and key layouts. So is there any way that i can edit a Nif models on 2.8 and then export it on 2.49b? since 2.49b exports the nif files perfectly with no issues or errors.

Thanks!

Yes ,download blender on  2.48 version create the nift and edit .later  with 2.49 import the file and export.
 https://download.blender.org/release/Blender2.48/
Or use 3dmask to edit the nift(with this i dont need blender but if u can use take the nift  and in 2.49 to import /export

yeah, thats how I did it, it seemed to work but I'm not to sure about the technical side though.