The VIP function serves as a tool to mark your top supporters. Once one of your followers has VIP status, their messages will be displayed in their own inbox.
You can also use the messaging filter to only see VIP's messages.
To check where you can locate your top supporters and grant them VIP, please head to https://fansly.com/creator/topsupporters. To VIP someone, click the 3 dots to the top right of their profile and select "VIP User".