Okay, I just dug out the voltmeter, and measured 2.5V between VDD_IO and GND on the WT32i module itself, when supplied with 5V over the USB cable. On re-review of the schematic, this makes sense, as there is a honking 2.5V regulator onboard for this purpose.

So if one plans to supply the board that way (5V to the 5V pin, resulting in 2.5V at the VDD_IO pin), then the resistors for the serial port will need to be different than what I suggested above: just make them equal in value. So a pair of 10K resistors, or 15K resistors would be ideal. This would split the 5V logic levels in half, to 2.5V. Perfect.

The idea of a large K value here (as opposed to say, 1K-1K) is to prevent wasteful current draw (Ohm's Law), thereby saving power and keeping the Arduino and WT32i cool.

If powering the WT32i with 3.3V, then the 20K/10K pair is still correct.

Cheers