2.4 - VS [U3 Datasheet]
The VS terminals are designed as outputs for the internal supply voltage (nominally 5 volts). This will be the voltage provided from the USB cable. The VS connections are outputs, not inputs. Do not connect a power source to VS in normal situations. All VS terminals are the same.
Related specifications can be found in Appendix A.
For information about powering the U3 from a source other than USB, see the OEM information in Section 2.12.
What if some VS terminals are not working? We hear this every so often, but since the terminals are all simply soldered into the same VS plane on the PCB it is almost impossible for some to work and some not. Almost every time the actual problem is improper use of the screw terminals. The only guaranteed connection with the screw terminals is when a conductor is securely clamped inside the terminal. For example, touching DMM probes to the screw head of a loose terminal rarely provides a valid contact.