You would need a multi-layered shader and a way to manipulate the position, rotation and scale of the decal's UVs. An example: https://forum.unity.com/threads/iphone-shader-simple-texture-with-decal-2-uv.30629/
I'm working on some objects that are low poly, like weapons and clothing around 250tris. Is there a way to add decals without using vertices? Do vertices even matter? I'm imagining it being like vertex painting. If I made a game in Unity where the user can put stamps on their clothes or weapons, how could that be done? Any…