I am having a bit of trouble with the API. I put the vault.jar in my plugin folder and added "depend: [Vault]" to my plugin.yml. However, java is not creating any kind of library from the jar (should it?), so all the imports can't be found. Do I need more than 1 jar to use the vault API?

you need to add vault.jar as reference to the project in your IDE.
In Eclipse: Open Project Properties, select 'Java Build Path' at the left, select Libraries at the top, select 'Modulepath' in the center. At the right click 'Add External JARs', now select the vault (or vault api) jar

Now my problem is that when I try to add the vault project (Called "master-vault") there's an error. It said that vault failed to create a JAR using ANT script... or something like that_?_ The error here seems to involve the "master-vault" being in the wrong format.

Other:
I got the "master-vault" at https://github.com/MilkBowl/Vault

@Lightning11wins I have no idea what you're trying to do. Are you making your own plugin? if this is the case you shouldn't be interacting with any of the source from github, and you should turn your questions over to the bukkit or spigot forums on how to develop a plugin with dependencies.

It sounds like this is the case, and you're just unfamiliar with using maven or gradle to build a plugin.

I suggest asking your questions on: https://www.spigotmc.org/forums/spigot-plugin-development.52/ - the community there should be able to help you out with getting a project setup.

I am just trying to add vault as a project and vault failed! (See above error) Is this a problem with what I did or vault?