I don't think passive HDMI-VGA adaptors are a thing, they would never work. You definitely need an active adaptor, but I can't help but think the one you bought actually is active.
The RX550 only outputs a digital signal. Your going to need an active adapter to convert the digital video signal to analog for your monitor's VGA connector.
Hate to break it to you, but I don't think you can run dual graphics that APU and an RX 550. Unfortunately AMD's page on dual graphics compatibility seems to be down right now so I can't confirm.