Not sure if this was available at the moment of writeup, but now it `clearly` states in datasheet "Connect to VCC to input external power when 3.3V power supply, connect to 0.1uF decoupling  capacitor when 5V power supply". This pin is an internal 3.3 voltage reference for USB operation. Therefore either internally generated or tied to external 3.3v reference, whenever possible

I received 10x wch330n chips yesterday, but in SOP8 format!

I am now hunting to see if I can find them in SOIC8 format, which is bigger to solder, as they appear on this HD article: https://hackaday.com/wp-content/uploads/2018/10/ch330.png

Will try to see if I can embed them in a simple USB plug, or just solder them on a stripboard and solder some wires for a salvaged USB plug.

The V3 test results are really useful - thanks for sharing!

I noticed that some vendors offer a "pin compatible" CH340N. Have you come across that chip?

Nice, thanks! I think you should have drawn more attention to a compelling feature of this chip, the built-in oscillator, which saves quite a few components.

On the H2O2 from Tindie, the V3 pin is not connected to 3.3V but only decoupled to GND for 3.3V operation. http://h2o2fromh2o.oregonsouth.com/Tindie/H2O2USBCH330N33/H2O2USBCH33033SerialProgramerSchematic.pdf

Is connecting V3 to 3.3V really required?

Hi there! You're right. When using 3V3 as VCC, it can simply be decoupled to GND with 100n. I dedicated some write-up to this exact topic but didn't do so clear enough.
Will edit and make sure it's unambiguous this time...  

Thanks for the clarification!