I personally think, While Interplay still has the rights to the mmo, and that Bethesda is busy with them. It won't have a problem as is. I think Bethesda would be the one who could shut this mmo down, As they own the IP. And Bethesda obviously doesn't care much for the old fallout games. So as long as we don't use anything from F3 and FNV as well as Interplay's MMO lives I think it will be fine. Unless the devs decide to earn money or whatever in which case Bethesda might get pissy.
But from their view atm, it's just a fanmod of the old games and isn't detracting from their sales atm.
However if the devs do get a legal notice. I doubt they will continue this. And I really really doubt Bethesda will llet a fan mmo live.. I mean look Interplay has a license to make a mmo and Bethesda don't want that either. (probably because they want to make an mmo now)