I think FFC chose to make the character of Vincent an illegitimate son for dramatic effect to make a point about Vincent's character.

Remember when Zasa is in the meeting with Michael and Vincent, Zasa turns to Michael and tells him that all bastards (illegitimate children)are liars, and that Shakespeare used to write about that kind of thing. In tradtional drama and literature, "bastards" were always portrayed as hot-blooded, cunning and often evil characters. This harps back to the ancient idea that the manner of your conception determines your adult character (ie, an illicit yet passionate affair would produce a person with a hot temperment).

So I think FFC used this traditional dramatic device and coupled it with the fact that Sonny's temper would have passed on to his son, to create the character of Vincent.

What Turnbull said is also true - Michael may have felt an obligation towards Vincent and this would be emphasised by his being illegitimate, he has been hard done by and has been partially isolated from the rest of his family.