you need to use dnspy to inject the code you made into the Assembly-CSharp.dll file found in Vessel Tactics\Vessel Tactics_Data\Managed; after doing that, you need to create a modded character yourself using the examples found in the "Mod Example" folder
I'm talking about this program, it's what I mentioned in an earlier post https://github.com/dnSpy/dnSpy/releases
it's a tool used for code injection / extraction, but you're going to mainly be worried about the injection part here since you need to edit the file I mentioned to get the code you made included in it, here's a basic guide on how to use it:
dnspy is required if you want to do any modification of the code itself, but there's another reason why I told you to use assetripper as well; technically using only dnspy to get the code could work, though I mainly told you to use assetripper with it as well since it gives you a version of the code that is more legible compared to dnspy's version of the code that it gives you