Everytime you unplug and replug the USB cable from display unit, or unplug the USB unit from the VDM it may recreate another module address. It may duplicate, change or create a faulty new address. Its dumb that way.
Heres what you do, but its a pain in the neck, and you will have to reprogram everything.
Connect only the display unit to the EB8 (it may be easier to mount everything first, connect everything, and then just disconnect the USB cable from the VDM). Then reinitialize everything under: "System -> Init Memory -> Init All." When it reboots, do not unplug any USB cables. First get the network going if you are using it. Then update the EB8 module. Then rename & assign the EB8 outlets. Remember with apex every outlet has to have a different name. Do not rename "outlets" the base unit makes for e-mail notifications, etc, only rename the 8 controllable outlet on the EB8. Now plug in the VDM USB cable. Restart the apex (do not reinitialize). Now update the VDM and change it to Sol blue if thats what your using. Then you can configure the VDM outlets, profiles, etc.
In the future if you add more EB8s, I would reinitialize again to be able to add 1 EB8 at a time in order of priority. Then I would add VDM last, or wxm, PM2, etc last.
There is a different way to change the module addresses without disconnecting everything. But you have to use the network and be good with telnet commands. Call or email neptune systems for this.