HDMI to VGA with USB-C VGA adaptor on 2015 Retina Macbook I have one of the new Macbooks on the way and have also bought the Apple USB-C Digital AV Multiport Adapter with HDMI to go with it.

If I want to get VGA with this adaptor, rather than buying the same adaptor but with VGA, I want to use an HDMI to VGA adaptor such as this one.

This adaptor has a USB power in if the HDMI out does not have sufficient power output. Will the Macbook adaptor's HDMI port have sufficient power to make this unnecessary or will I also have to use the adaptor's USB port when using VGA?
You won't know until you actually get it and test it.  

HDMI specs say you should get 5v at 50mA.  That's not much to start with.

You can read about this more from here.  You could get more power but it would have to have MHL (mobile high definition link) support.  That will be able to give you up to 40 watts.

Now, I looked on the Apple site and it appears that the Adapter you selected isn't very highly rated.  I have rarely had success with genuine Apple adapters and instead had to rely on 3rd party adapters to get a reliable results.

You may want to save yourself the headache and just get the VGA adapter and be done with it.