Dealing with USB VBUS on a 3.3v board
According to the reference manual for the STM32F205, page 950, you want to do the following:

First, you want to disable the VBUS sensing option. This is done by setting the NOVBUSSENS bit in the OTG_FS_GCCFG register. In this case VBUS is considered internally to always be at VBUS valid level (5 V), so you don't need to run a trace from the VBUS pin on the USB connector over to the STM32F205 at all.

Note: you can also skip this step, and tie VBUS from the connector to the VBUS pin on the STM32F205, keeping the VBUS sensing capability (so, for example, you can tell if a USB cable has just been plugged in).  Just don't implement the voltage regulator as shown in Figure 351 of the Reference Manual so your device doesn't try to power itself off the USB.

You can also disable the OTG capability and force a peripheral-only mode by setting the force device mode bit in the GlobalUSB configuration register (FDMOD in OTG_FS_GUSBCFG) to 1, forcing the OTG_FS core to work as a USB peripheral-only.