STM32F4 VCP transmit/receive data
In my VCP project:

file: usbd_cdc_vcp.c

uint16_t VCP_DataTx (uint8_t* Buf, uint32_t Len);  &lt;-You call it to send data from STM to PC

and 

uint16_t VCP_DataRx (uint8_t* Buf, uint32_t Len) {
  // Called when data from PC to STM is available.  

  // YOUR CODE IS HERE

  return USBD_OK
}