Excited to get a VMU tonight to finally be able to save games! Yeah! …oh wait, no. 
VMU was advertised as new, looks new and unopened (likely new old stock). Purchased here on ebay. Opened it, pulled the battery tab, hit the reset button, able to set date, times, etc. I plug it into the controller and…nothing. Controller doesn’t work until I pull out the VMU. VMU needs to be reset to work again. I replaced the batteries in the VMU with brand new batteries and still same thing. Tried both VMU slots, tried wiggling it around.
This is a used DC, don’t know pedigree, and also came with a 3rd party controller, which some search results say is a likely culprit of frying a resistor. Tried the 3rd party controller and it has the same results for both VMU slots: controller is unresponsive until I remove the VMU. Tried OEM controller on controller ports 1 & 2, same thing, nada. Decided not to proceed testing for ports 3 & 4.
I’ve barely used the DC and when I do it’s usually been off power for a while. However, while testing tonight I noticed that if I turn it off but keep power supplied to it, when I turn it back on after only like 2 minutes it prompts me to set the date/time again (this is the console not the VMU). This indicates to me something is wrong inside the console.
I don’t have time to tear into it tonight, or search around online much more, but interested if anyone has recommendations on where to start looking. I plan to clean the controller VMU contacts, might even do a whole controller teardown/clean. I’ve found some forum posts similar to what I’m experiencing here and here. I will likely also open up the console and take a look around. Unfortunately I don’t have any other OEM controllers to test. I may stop by some local game shops to see if they have any after cleaning mine, assuming that doesn’t fix it.