Alright, I'm back after looking through more Threads. It looks like the Vanilla issue is quite common, but has seemingly been resolved.
This is the Thread that seems to be most help right now - http://forums.technicpack.net/threads/compiling-a-modpack.40032/
I read through a bunch of comments and such and it looks like one of the most common problems is the modpack.jar having been named incorrectly. For example, it needs to be named modpack.jar and not modpack.jar.zip
Now, I already was making sure that it was named modpack.jar, but according to said Thread, it can still have that extra .zip extension without you actually seeing it.
So in order to fix that, we are supposed to change the computer settings to allow us to see that extra extension and rename it properly.
It seems like maybe this might work.
I think I'll go give it a shot. If it works for me, I'll post here. If not... I'll post here lol