Houndawg is right!
If doing GREAT vocals is your MAIN thing, then you need a decent mic, and a decent preamp/channel strip to feed a WONDERFUL sound into your A/D converter.
I would worry more about your mic/ mic preamp, and less about your A/D converter.
These days, the quality of A/D's have really ALL improved and the premium converters don't stand out as much as they did 10 years ago. Today you can get very respectible quality without having to reach for premium brands....that's my opinion.
I think something like a Focusright Scarlette 18i6 would serve you well, and spend the money you save on a Unversal Audio LA-610.......or something similar.
It would stretch your budget, but think about it this way:
10 years from now, when your computer is a dinasour, and the Focusright is on it's last leg, the UA LA-610 will be just getting warmed up. :) Same thing with a quality microphone.
I have used two RME interfaces for the last ten years. I never regretted the decision to buy them. they are great, but my recordings didn't sound professional until I invested in a quality preamp and microphone.
I just purchased a Focusrite scarlette 18i6 and it sounds just as good as my RME's, which I bought 10 yeras ago