Mixed Authentication support was added in 2019 and not available in 2017.
I would convert the vault to use PDM login type to detach the AD authentication, but it'll be nice that the login names match the AD. Once you get up to 2019 OR an environment that allows you the permissions you need, you can convert it back to Windows Login type. Convert to Windows login is a bit more work but not so much that it's a prohibitive process. Instructions to do so are covered in the solution S-043890 in the SOLIDWORKS Knowledge base. I imagine the process is very similar to convert to Mixed Authentication in 2019 and beyond.