A forum is basically a multi-user blog with comments. For many cases, that's all you need. If you need something more like vBulletin, then you should probably just use vBulletin.
I can't see anyone coming up with a forum system like what already exists in the PHP world. These things were created when forums were big and have massive man hours of work to build on. To create something like that from scratch today probably wouldn't be worth the effort. If it were worth it, then there would be more available in different ecosystems.
