- DP > HDMI is passive, it's the same signal in different connectors.
- HDMI > VGA requires digital > analogue conversion.
To use this monitor I have to connect the Minidisplay > HDMI > VGA but it doesnt seem to work.
Should it work?
It depends mostly on the specific HDMI>VGA device you're using. If it's just a port adapter (cheap, passive, and useful for some non-standard applications/devices only) then no, it shouldn't. A converter (more expensive but providing the required digital>analogue signal conversion) should work but may not.
Generally it's better to have just a known good DP>VGA converter even if, technically speaking, there's nothing wrong with your configuration, assuming that the last device is actually a converter.