Hello,
1) "I would like to use P6 as UART connection and another USART connection using any of other USB port(P5, P9 or P10)".
Yes, use other USB port is impossible, while there is still other UART we can use through jumper you can directly use, about detail please check schematic of LPCxpresso55s69.
2) About example for USB CDC VCOM, download SDK, you can find it.
Download from below link, choose LPCxpresso55s69.
https://mcuxpresso.nxp.com/en/select

BR
Alice