For Fallout I played vanilla first. I like to do that, but in the case of Oblivion I hated the scaling so much I played with a mod to help with that(a very basic Fransiscos, as this was when OB was still new). Morrowind and Fallout I think are better done out of the box than OB so you should have no issues. After words though you got to check out the mods, I love Beth they make modding so easy for the modders so there is so much choice for us gamers. With mods Oblivion is easily my favorite game ever, I don't know if I could say that if there were no mods. As for Fallout I don't seem to use or want to use nearly the same amount of mods as OB, both because I think the quality of mods is not nearly as high and because the base game seems better thought out. Good stuff takes time, look at how long some of the great Mods for Morrowind took to come out after the games release. Other than FWE and MMM and Darn there arent any mods I really could not play with out, unlike OB which has about a 7 or 8 I fined essential.