1. Mic..u mentioned u have a usb mic .. Usher most likely used a mic that is $2000+ with really expensive preamps n converters.
2.EQ.. you have to EQ because i doubt ur gettin the results u want with that mic. Learn how to eq properly.
3.Enviorment... DO NOT RECORD IN A CLOSET IT WILL MAKE IT SOUND WORSE!!! it will just color n muffle the sound . u will not achieve big open vocals in a closet. Get ur room treated with some acoustic panels or some auralex foam.... And it was recorded in a million dollar studio..
4. And ur mix wont sound as good as his cuz he had it tracked out n mixed with the vocals and ur two trackin it.. my 2 cents..