I did a good amount of research on aftermarket battery grips for the D810 (versus buying the insanely priced Nikon model) and settled on the Vivitar version. Note: I'm using only genuine Nikon batteries. What I'm experiencing is that when I put the battery grip on the camera, it works and I see on the display that there are 2 batteries connected (yes they are fully charged). BUT what happens when I press the "info" button is that all of a sudden the low battery indicator flashes and the camera won't work. Removing the grip's battery doesn't help. I have to remove the battery for about a minute and then when I put it back in, the camera will turn back on. The easy solution is to not press the "info" button...but there's gotta be more to it.
I need to run through more tests to see the different scenarios it might occur, but that's finding after 1 day of shooting with it.
So this is part information for you all...but also a solicitation for info/feedback if anyone has experienced this before (whether or not with a Vivitar grip) and how they resolved it. (Note: yes the battery is well seated in the battery grip tray, yes I'm using only fully charged, original Nikon batteries, yes the grip is full seated onto the camera).