How do I configure WEMOS D1 R2 Vin pin to output max voltage?
You cannot. You are misunderstanding, what that pin is. Vin means Voltage In. It's a voltage input, connected to a voltage regulator, which will regulate the provided voltage at that pin down to 3.3V for the Wemos to work with. It is not an output. The regulator cannot convert the 3.3V from the Wemos to 12V. That is not, what it is build for.

To get 12V you either need a separate 12V power source, or you need to buy a stepup/boost converter, which brings the 3.3V (or 5V, if you are powering from USB) up to 12V. For the latter, be sure, that the converter and the source, where you get the input voltage of the converter, can provide enough current for your use case.