Frankly it sounds like these vendors are having cash problems. All of those excuses are basic Accounts Payable 101 excuses for why the bill has not yet been paid.
It's harder to steal from Accounts Payable then it is from payroll. First of all, there has to be someone ELSE involved in the scam.
The Accounts Payable department at the little hospital I worked for did manage to embezzle over 100K from the hospital. They did it through the hospital's system of what they called "patient refunds". Which is when the hospital cut a check for a "patient" that paid their bill in advance and the insurance paid the same bill later.
Some of the AP people got some friends' names and began issuing patient refunds to these friends. This also required cooperation from the patient accounting department as they had to approve patient refunds.
The AP friends cashed the checks in their own name then likely split the checks with the AP theives.
A department manager could easily put a ghost on the payroll. They could turn in all paperwork for an employee to the HR department. Somehow it could slip by. Just locate the people who don't do their jobs and make them your contact.
"Sally, I interviewed this nurse yesterday. She can't come in because she's on 24 hour shift at another hospital but I have all of her paperwork, including copies of her ID's. You know Doctor Bones needs an operating room nurse immediately so can you push through the paperwork so she gets paid?"
This is how most fraud gets committed. By thieves smart enough to find the holes in the system and by locating employees who are stupid.
Of course the employees' paperwork would request a direct deposit of the paycheck, said checking account provided owned by that very same manager.
Turn in timesheets every week and boom, a net paycheck gets deposited right into my account, very convenient.
The less people involved in a scam the easier to get away with it.
Of course there's a million ways my above scenario would not last forever. But hey, they rarely do. Extortions and scame are usually caught. It depends on the quality of the employees involved all around on how long it takes to catch them. The intent of these scams is not that they go on forever. Get the money and run is the logic.
If someone, somehow, doesn't notice timesheets submitted for employees never seen, or the Feds don't finally get around to notifying payroll that the SS# for this person is not valid, or some other employee stumbles upon the truth, well who knows, an entire city could get flooded with only ghost cops on patrol who then all conveniently abandoned their posts!
Yup, I think those ghost cops were assigned to cover during the Katrina aftermath because, a)no one expected the flooding and b)THINK OF THE OVERTIME!
It's how it works, ladies and gems. IT's how thieves think.