#include is basically the same as copying and pasting. Your example code is one plugin, just split up into multiple files. That's why you're getting an error about defining the same function multiple times.
If you actually make them separate plugins, you can use a global forward like in the code you gave. You'll probably have to make natives (like in the article linked by the previous reply) since you can't directly call another plugin's functions.
The simplest change would be to give them different names like
OnVipMenuEffects and
OnVipMenuRevive and manually call them instead of using a forward. If you add new files, you'll have to remember to update the functions you call.
Another way is to use a private forward (see
https://wiki.alliedmods.net/Function...Mod_Scripting)) to have each file register its own callbacks. This isn't very different from the previous suggestion, though, since you'll still have to manually do that registration/call some initialization function per file.