You need to make a batch file, or rather two batch files: one to switch to vanilla, the other to tmodloader.
ETA: I wrote two quick ones for myself, which I've attached. They assume that you've already backed up your vanilla Terreria (as Terraria.exe.BACKUP) and the tModloader version (as Terraria.exe.tMODLOADER), also that you;ve got Steam installed on your D: drive (so you'll need to change that if appropriate). To use just double click the version for which you want to run and it will copy the appropriate .exe and launch it.
ETA2: Okay, apparently you can't upload .bat files here. So, make 2 new text files, rename them to whatever you want, making sure to change the extensions to .bat, as follows:
Code:
copy "D:\Program Files (x86)\Steam\steamapps\common\Terraria\Terraria.exe.BACKUP" "D:\Program Files (x86)\Steam\steamapps\common\Terraria\Terraria.exe"
start /d "D:\Program Files (x86)\Steam\steamapps\common\Terraria\" Terraria.exe
Code:
copy "D:\Program Files (x86)\Steam\steamapps\common\Terraria\Terraria.exe.tMODLOADER" "D:\Program Files (x86)\Steam\steamapps\common\Terraria\Terraria.exe"
start /d "D:\Program Files (x86)\Steam\steamapps\common\Terraria\" "D:\Program Files (x86)\Steam\steamapps\common\Terraria\Terraria.exe"
As written, the code assumes default Windows 7 paths, so of course you'll need to adjust those if your install is somewhere else.
Use at your own risk, I make no guarantees.
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Have you tried deleting and redownloading the Calamity mod? Have you tried running the game with the Calamity mod disabled?