USB protocol PIC32mx
Q1: No. The VBUS pin is used when your PIC32MX is itself acting as a device and it tells the PIC32's USB module when it has been connected to a USB host (like a PC) by detecting the nominal 5V which should be present.

Q2: As the manual tells us, the VBUSON pin is "USB Host and OTG bus power control output". In other words, this pin turns the Vbus 5V output from your board on/off when your PIC32 is acting as a USB host.

To address a question which you haven't asked ... your PIC32 must be operating as a USB host in order for your FTDI USB UART to be of any use. This means that it must be configured correctly and running a USB host firmware stack.