I noticed that when I try to buy a bank slot using the plugin Global Bank and I do not have enough money, I will get a negative balance. I have been told this is a problem with vault and would like to have this fixed as it could cause huge problems on my server, Thanks.

Fixed, although essentials should really be doing hasEnough checks before doing withdraw operations.

Essentials has a permissions node essentials.eco.loan which overrides the hasEnough checks. Make sure you don't issue out that permission.