I am doing my own development board using LPC1343.I gone through the application note AN10986.I noticed the topic "Automating USB ISP".In that they described about USB softconnect. Now I am designing the board,a question arises whether is it necessary to use that softconnect or manual working is enough?....and also how much voltage(minimum) should be supplied to the VUSB pin to detect as USB ISP.
You don't need to use the softconnect if you don't want to. The manual method will work perfectly fine.
The VBUS pin is usually set at 5V as that is the standard voltage for the USB power lines. The controller will be looking for that voltage.