The VN in this case should still work. it's not an MDB model, so it still sends out pulses for credits instead of the communication protocol.

Unfortunately, it's all guessing with the statement 'they don't work'.
Does it power up or not?
Does it accept a dollar or reject it?
Does it accept a dollar and just not give credits?

More info is really needed, or else it's just a 'check everything!'.

No, nono.

Here's a summary of hte model number-
It is broken down into:
"VN2400R-U2M-XX"
The first two are the usage. 'VN' is 'Vending'. 'AE' is Amusement. Vending models offer coupon coding, and come in 24v MDB models. Amusement models don't.
The third digit is the 2xxx model range. Only 2xxx models will fit pin doors.
The 4th position can be a 3 through 8, and are called (and accept [$ bills / directions]):

3: Economy [1, 2 / 2]
4: Reference [1, 2, 5 / 4]
5: Economy Reference [1, 2, 5 / 4]
6: Premium [1, 2, 5, 10, 20 / 4]
7: Recycler Ready [1, 2, 5, 10, 20 / 4]
8: Reference Expanded [1, 2, 5, 10, 20, 50, 100 / 4]

The 5th position is the bezel. Only numbers 1 or 4 will fit in pin doors. Faceplates are, for the most part, interchangeable.
The 6th position is the voltage. WPC Machines (and maybe 2000-2010's Sterns?) only accept 110v, so this digit must be a '1'.
The 7th position, if present, is an 'R', and means it came with a Recycler unit. Pin games, and AE models do not support Recycler units.
The first item after the dash is either 'U' or 'D', indicating Upstacker or Downstacker.
The second position after the dash is the size of magazine that the unit was shipped with. Interchangeable, and various sizes can fit depending on game maker.
The third is the harness shipped with. Usually, an 'E' is required, the "9-pin VFM Style / Single Price" harness.
The final 'XX" is the country code. Usually, 'US'.

Now, as you can see on the label - and remember, pin games can only accept pulse interface - dip switches #7 and #8 both need to be OFF. You also will need to check your harness - make sure the proper pins are connected up. It's recommended to have it 1 pulse per dollar, as well, but will require the correct coin input to be wired to, and the game knows the value of said coin input.

If it's accepting bills, but not registering credits (again, need more than "they don't work"..), then it's possible that the serial line is being output and not the pulse wires. (In other words, wrong harness!)

If anyone is interested, I have a full document spelling out the model numbers available, and I'd be happy to forward it to anyone interested.

Sounds like either:
(1) The 'Enable' pin is being held inactive, preventing the DBA from accepting anything, or
(2) The DBA is going into an error mode.
Look on the back of the DBA, under the magazine, after it powers up. There will be a red light. Count the number of flashes, or if it's solid. Likely, since it's happening on all your DBAs, it's #1 above.. likely you have the wrong adapter for pulse output..