Sorry, You are correct.
I was thinking of the regular RX 5500M GPU but you have a RX "PRO" 5500M GPU which seems to be a OEM GPU made for Mac PCs.
The Unofficial Boot Camp driver link I posted, the author takes the latest AMD drivers and modifies it to work in a Windows Boot camp environment. As per its Compatibility Chart concerning the latest driver it has to download:
KNOWN ISSUES from the screenshot above:
Users of MacBook Pros with 5600M, 5500M, 5300M can suffer from thermal throttling which leads to reduced fps in longer gaming sessions.
Please google 'MacBook Pro VRM mod' and follow the instructions for a permanent fix (warning this is for experts only!)
According to TechPowerUp Specs on the Pro 5500M it does support Vulkan 1.3.
You can open a AMD SUPPORT Ticket but most likely they will tell you go to Apple Support. But maybe they might be able to let you know which Pro 5500M driver support Vulkan 1.3.
AMD SUPPORT: https://www.amd.com/en/forms/contact-us/support.html
EDIT: From past experience with Users and Mac PCs - AMD SUPPORT doesn't support Mac PCs with Windows Boot Camp and they will refer you to APPLE SUPPORT. But then APPLE SUPPORT will say to go to AMD SUPPORT for Windows Boot Camp drivers issues.
But, hopefully, they at least might be able to let you know which Boot Camp driver will support Vulkan 1.3 for your GPU card.
I believe that AMD supplies Boot Camp drivers as a courtesy.
Also maybe try contacting the author of the Unofficial Windows Boot Camp website. Possibly he will know if the latest Pro 5500M driver supports Vulkan 1.3. No harm in asking.