Just heard back from vfs on my email query.
This response below is similar to another thread here from 2023. Seems like vfs doesn’t want to fix it. Oh well!!! 
Thank you for contacting the VFS Global Indian Visa, Passport & OCI Application Center Help Desk in the United States.
Please be informed that while completing the registration of renunciation application on the VFS website, you are required to provide the details as per the US passport, wherein it is specifically asking for Indian passport details, there you are required to mention Indian passport details.
Also, if you are not able to enter your full name while registering on the VFS website, then you may mention the name as less than 13 characters and you may write a cover letter stating the scenario, why you have entered the short-name, and mentioned that this is a technical issue on the VFS website.
Please note that you may include a cover letter while submitting the application and documents to the VFS Application Center.