CONFIG_DEBUG_VF_UART is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Vybrid UART

configname: CONFIG_DEBUG_VF_UART

Kernel low-level debugging port
└─>Vybrid UART
In linux kernel since version 3.13 (release Date: 2014-01-19)  
Say Y here if you want kernel low-level debugging support
on Vybrid based platforms.