i'm still curious to see the mic pre plug-in, though
I think the OP isn't talking about gain, they are talking about a magical effect (software or hardware) that will sprinkle fairy dust on the vocals and make them sound professional.
Nightshivers: It's garbage in, shined up and polished garbage out, here.
You need a good performance - in tune, on time, and with emotion that works with the song. You need to record it with a minimum of unintended distractions, like P popping, sibilants, volume and tonal variations from moving in and out of range, overemphasized proximity effect, and the like. Using a pop filter and schooling the vocalist in technique (as well as the mic selection) is important here.
Compressing the #*$& out of a vocal is a technique that may make it sound like certain types of professional recording, but used in the wrong place will make it sound lifeless and squashed.
The room sound is a major component of the vocal sound. You can choose between a live or a dead space, but it should be free of resonances, external noises, and distracting echoes.
The microphone has to be right for the voice and the song. Not that it has to be a $2000 mic, but that it has to reinforce the qualities of the voice, and give a tone colour appropriate to the song.
In short - a recording sounds professional because it was made professionally--- which means with a multitude of recording choices made though experience, disciplined experimenting, and good, hard listening.
There is no "make it Pro" button you can press. No "magic Ratio".
A good place to start is at the library, and read as many back issues of Sound on Sound, Recording, EQ, Electronic Musician, Mix and other magazines as you can. They are full of case studies, how-tos, interviews with engineers and producers, and equipment reviews and comparisons.