Why can't VDD-USB be tied to voltage derived from USB VBUS, even when VDD is OFF? If they need to be tied together, than the USB will disconnect every time VDD is OFF and will need to be re-connected. It should maintain this connection when VDD is OFF.

[JKlec]

Does the STM32L072VBT6 or STM32F072VBT6 not allow for the USB connection to function, even when VDD is OFF?

[CYANG.1]

Normally, VDD_USB is used as embeded USB PHY inside STM32. if there is no power on this power pin, USB feature won't functional.