most of the achievements are in the included campaigns which don't let you enable other mods to begin with

there's one for doing the tutorial: also doesn't let you enable mods

there's one for playing 10 ranked multiplayer games: ranked doesn't let you enable mods last time I checked

the only remaining achievements for multiplayer are for hosting a lobby with the dlc enabled(if you have it), using the moonshot commanders, playing 10 multiplayer games, and winning capture point with your blast doors open. these can all be done with workshop mods or not

how do i disable then those mods

forts applies dlc(Tons of guns, repair station, portals, moonshot, high seas, control ammo) in the same way as included mods (like fast build, unlock all devices) and workshop mods (like tanyas mod).

unless you play workshop maps that force enable mods (they'll be darker in color and no [x] to remove them) or manually enable mods there's no need to remove anything to play 'vanilla'.

multiplayer by default only enables tons of guns and repair station (an included expansion/free dlc). Most people I've see consider the tons of guns/repair station (ToG) expansion to be vanilla(and I do as well), if you still want to remove those I've seen people refer to that as classic or old forts sometimes if you're hosting just click the x on them in the multiplayer lobby .