Note: Firing up certain mods might cause your vanilla save files to get overwritten or temporarily disappear. If you run into this issue, follow these steps:
C:\Users\<YourUsername>\AppData\Roaming\SlayTheSpire2\steam (Tip: You can just press Win+R and type %appdata% to get there faster).
[modded] folder, and copy/paste all those vanilla files straight into the [modded] folder.