Use the Imbedded Interface Points (IIPs) to override existing behavior or add specific internal requirements to the supplied by existing Framework functionality.
You can validate passwords according to specific requirements, check the authority of a user to a certain VLF object, have your own program validate the VLF-ONE sign on and many others.
In VLF-WIN the IIPs are shipped in component UF_SYSTM (reusable part).
VLF-ONE the IIPs are shipped in component UF_OLOGON.
Read carefully the extensive comments in the source code of these components.
Make a copy of them – do not modify the shipped ones. See Create Your Own Imbedded Interface Points (IIP)