Hello Sam!
At the moment, ArcGIS Online does not have a vector tile layer legend capability. We have several customers asking for this capability and we have an enhancement logged in our system:
[ENH-000099289: Please enhance ArcGIS Online to generate legends for vector tiles automatically similar to how raster tiles do.]
The enhancement is currently in an "Under Consideration" status. I'd recommend that you give us a call here in Support Services and request that a case be created on your behalf so that you can attach your account to this enhancement, thus adding support for the idea to be put to action. Provide support with the above enhancement number and they can assist with that request.
We look forward to hearing from you in Support!
Best regards,
Rachel
Esri Support Services
Rachel Guttmacher
ArcGIS Online Technology Lead
Esri Support Services