I heard Vial has some cool feature like support for rotary encoder and I will switch to Vial soon.
Vial is decentralized: with Vial you do not have to submit your keyboard as a pull-request to any third-party repositories before it can be used in the GUI; instead, the keymap JSON definition is stored within the keyboard firmware and is retrieved at runtime.
if the JSON is saved in the firmware, meaning firmware size of Vial is bigger than VIA, isn’t it? though that would enable Vial software to detect the keyboard without using “Import Keymap”, am I right? 
I already have the environment to build VIA firmware, is that ok for Vial?
I’m using MSYS2 MinGW 64-bit on Windows 10