OK Ace, you did not tell us you didn't have a CFast card in the camera. We are good, but not that good
Bad news: All the pro video cameras, including BM cameras, I have ever used would not "record" without media (disk, tape etc) in them. The only exception I can think of, is a real film camera, you could run it without film (kinda hard on th camera though).
Good news: If you want to record only on a the VA4K, you are in luck, there is a work around. When you are using the camera in a tripod, just touch the VA screen "Record" button to start a recording (I do this with my Micro Studio camera and AF100), quicker than reaching for a camera record button. When shooting with the camera handheld or a better idea, on your shoulder, Connect the camera's side grip connection cable to the VA4K LANC plug (you might need a short ext. cable) instead of to the camera, then, pressing the red record button on the grip will start/stop the recording on the VA4K. The grip is actually a LANC remote, and the plug on the camera it connects to is a LANC input.
Now, you can only record widowed 4K and down, ProRes or DNxHD recordings, not full open gate 4.6 or Raw files. Also you can not do over cranking (limited to HD 1080p60 max fps or 4K at 30fps max).
Good luck