First, you have to use an active adapter. That is, an adapter that converts the digital signal of HDMI to an analog signal of VGA.
Second, you HDMI-VGA adapter needs to be externally powered. We have made tests with several HDMI-to-VGA adapters. Adapters that are not powered by some other power source besides the HDMI port, behave erratically on Yodeck Playbox (or Raspberry Pi). They either don't work at all, or work for a while and then go on and off constantly. This is because the HDMI port on the Raspberry Pi does not provide enough power to the adapter. Adapters that are externally powered work fine.
For example, this adapter type does not work properly, but this powered adapter has been tested and works fine. If you are having trouble, try powering the adapter on its own PSU and not from the Yodeck Playbox (or Raspberry Pi) or the TV set.