Can I customize the VLF-ONE application logon screen?

Yes. The shipped demonstration system does this by using shipped example reusable parts UF_OHEADP and UF_OTRALP to customize the header and trailer parts of the standard logon:

Refer to the shipped source code for details of how to make you own customized logon header and trailer reusable parts.

To completely replace the logon dialog, refer to the source code of shipped entry point UF_OEXEC.