If you're just recording vocals, I'd assume you're rapping or singing over finished beats, then you have no use for an Audio interface. I went a couple of years without an interface just recording vocals. If you're going to venture into the realm of Audio Production, Audio Engineering, etc., then yes go for one. But make sure you do your research and know what you're getting into. If you're forking up the money for an interface you'd better also be investing in Studio Monitors, an XLR Mic, and some treatment. Again, if you're set on just recording vocals and the USB mic sounds fine for your needs, invest in a good mic stand, a shock mount, a mic isolation shield, and if you have loud computer fans buy some quiet components. Invest wisely, know what you're getting yourself into.