Where did you get that command line from? The command line (-pLPC4370 -vendor=NXP) is for programming an LPC4370.
If you have a working GUI Flash configuration, all you need to do is "Just copy flash command to clipboard" and the command line command will be in your clipboard.
EDIT: Also take a look at chapter 14 in the User Guide, where it describes the Command Preview which shows the exact command line and any setup you need to do, to run it.