STM32F103C8T6 5V pin description
That is a board with an stm32 processor on it, known as a blue pill, rather than an stm32 as a product.

The schematic for the board is http://wiki.stm32duino.com/images/c/c1/Vcc-gnd.com-STM32F103C8-schematic.pdf 

The 5V pin you mention is pin 3 of the header P4, connected to a net called 5v. There is then a regulator to bring the 5 V down to 3.3 V on the VCC3V3 net which is used for powering the processor, so yes, you can connect 5 V there, or use the USB socket to power the processor. The pin labelled 5V and the USB power pin are shorted together, so you should only drive one of them.

The schematic seems to use VCC3v3 and VCC3V3 interchangeably, and the same with 5v and 5V, I'm assuming they're the same nets, else the regulator is redundant.

You have a misunderstanding about pin VSS_5 from the datasheet, that's saying it's the 5th VSS pin, and should be grounded. All the VSS pins should be connected together, and connected to ground, there are many VCC and VSS pins around the processor to help with power distribution.