So I didn't fix the bug, but I found a solution to the achievement problem thing:
1.Enable cheats in launcher "configure" menu.
2.Load up your save.
3.Use console command cheatmenu. (You can learn more about it by googling "warband cheats")
4. Ctrl+T to see all npcs on the map.
5. Ctrl+left mouse click on the map to teleport to that location.
6.Find every vassal of the faction on the map and speak to them.
7.Find the option "cheat! like me" to get out of negative relation with the vassals.
8. Persuade those vassals to join your faction.
9. After the last vassal joins your faction, open console command and type nocheatmenu. save the game and exit the game. (I suggest doing that on separate save just in case)
10. Launch the game and disable cheats in configure launcher menu.
11.Load the game and wait couple of days. The faction will be defeated and you will get the achievement.
