Look at the second message.
I'm looking for a USB stack VCP (virtual com port) for LPC23xx / LPC24xx / LP serie s microcontroller The application provided by keil will work fine , but is v complex and takes too many resources of my MCU. I need the application should be very simple That do only the essential, rx tx data dcd rts cts nxp
That is your fundamental problem: USB is not simple!
But, surely, LPC23xx / LPC24xx has plenty of resources for this??
If you really don't want to burden your microcontroller with the USB, then you will need to use some sort of external USB "bridge" device such as: http://www.ftdichip.com/ www.silabs.com/.../default.aspx