Interesting approach to building a vocal room. Yes, it will work. Especially considering that most musical works these days will be mixed in with other instruments so much of the background is masked in the mix. Computer fans, your house air conditioner, and even the sound of your mouse sliding across the desk can be picked up on a good microphone. If you are using a gate on your inputs, odds are the only time you will hear anything is on the quiet parts of your songs. If it works for you, great. Not to be rude, but your "zero background noise" cannot be totally accruate. I'm sure you had to redo takes because someone slammed a door in another room or your furnace kicked on at the wrong time. Also, keep in mind that when you have a song where your vocalist is barely wispering in one bar and screaming in the next, you will have to do some crafty compression which will bring the sound levels of anything quiet in the background up to an audible level that will show up in the mix.
As for my studio, I recently recorded during one of the worst thunderstorms I have seen in a while and even in the quietest vocal sections, not even a hint of thunder or the rain beating down outside. When you walk in my control room there is a very noticable silence from just walking through the doorway, even distinctly noticable by those that know absolutely nothing about recording. Then when you walk into the vocal booth the silencing effect is twice as noticable. I have done some testing and I found that sounds under the 45 dBA level don't even register on any equipment that I have. I can turn my studio monitors on at a decent volume while recording vocals and not have to worry about the music or metronome bleading into the vocal track. In fact, the headphones the vocalist wears is louder inside the booth than normal conversation outside the booth.
If anyone is curious how I built it, it wasn't too hard. I think the sheetrock is the most difficult task. It's 4' x 6' in size. If you do a google one this topic you will find a lot of bad advice and lots of people telling you to use plywood. Before I built anything I read all of the engineering specs and found that sheetrock works best, is cheaper, but you have to install it in layers with a barrier or isolator inbetween. Use two layers of 5/8" sheetrock on each side of the wall and seperate the layers with "sound barrier" material (also known as limp mass vinyl) which is 1/8" thick. I lined the inside with studio foam with 100% coverage and bass traps in half of the corners. It should be air tight, and yes, ventalation will be an issue so put a (insulated) window in it large enough to get some light in but small enough to keep the sound out (glass transmits sound more than sheetrock does). Having a window will cut down on heat because you don't have a lightbulb, which is a heat source, inside the booth. Or, try one of those new LED lights if you need to. Either way, you need a window for visual communication with your vocalist. Don't forget about the ceiling! The ceiling and floor should also have a layer of 1/8" sound barrier material as well as an additional layer of floor board and/or sheetrock.