I'm planning on starting a new character, however I want to disable dawnguard at first so that vampires don't kill random npcs.
However after I disable the esm file Skyrim will crash at the main menu (enabling the file again stops the crashing)

Any ideas on how to fix this?

Often when you disable an ESM and get a crash on launch it means you have some ESP enabled that is dependent upon the Master file which you disabled. If you have some mods which are dependent upon Dawnguard assets this could be the source of your problem. You could try unchecking any ESP's you have checked one by one until you can identify the culprit.

Ah it was the Dawnguard version of the auto un-equip ammo mod thanks for the help