Hello there. If you want to make a waifu/VRM file or "VRoid Model" for the game, you need to go to "VRoid Hub", also known as "hub.vroid.com/en".
They have a program you can install for free to your PC, called the "VRoid Studio" program, where you could create your own VRoid for the "fattening game".
After you are done making your "VRoid" waifu, the Studio program will give you the option to convert your VRoid into a "VRM File".
If you have issues trying to make your VRM file, just look up a video on how to make a VRM file from a VRoid.
To answer your other question, I'm not going to share my VRM file, it would be better that you create a VRoid of your liking.
You could just download a "free" VRoid they have on the VRoid Hub site, but some VRoids are made to not be used in fetish-themed games, due to how other creators have edited their VRoids.
If you have any other questions, feel free to reply again. I hope you are able to create your waifu, or waifus to fatten up in the game!
Peace.