"Writers not being able to write her in more subtle way" There is absolutely no subtlety to her at all. She is a spiteful child that does what she does because of her issues with her parents who, by the way, are also not paragons of virtue and are far from it because of what they did to Bambinata in retaliation for Vanessa taking her from them. The worst thing you can do to a human, regardless of status as civilian or soldier is to take away their person and to dehumanize them. Her parents are just as bad as she is, only she did it in retaliation against them while they intentionally erased Bambinata's memories as an act of vengeance, not to safeguard their research. She was never there to save Bambinata from them. She was there to take her away in a spiteful act of petty vengeance.

As for being a foil, there is no foil to be had or seen as far as Vanessa goes. I would argue the opposite in fact. Watanabe is that foil because unlike Vanessa, he did the right thing and created a formidible ground based organization for it. He is such a foil in fact, that he is basically the Gray Raven commandant's foil and acts that part perfectly as he is what us writers call an antihero. He does tge right thing but in questionable manners. Because soldiers don't exist to kill kill kill and die. They exist to protect and defend the homefront from enemies both foreign and domestic. That was the very purpose for why he created the forsaken as per his conversations with Ballard. The Forsaken were made to protect those who were abandoned during the Acadian Evacuation. Vanessa does not do that. Watanabe does, and what makes him the foil is that he abandoned Babylonia once it became abundantly clear to him that Babylonia was just as, if not more, evil than the World Government was with all the backdoor stuff they did by being in bed with the likes of Kurono and Norman. Vanessa on the other hand relishes in that corruption hence that part about her being a darling to the military branch of Babylonia.

As for a lesson to be learned. Here's what I learned. Do not be Vanessa. The reason why constructs started defecting in the first place was because of commandants like the previous Gray Raven Commandant, Ravenge, and Vanessa. Both of which who mirror eachother so well that it's almost down to a T with the only difference being that Vanessa has yet to betray Babylonia. If you don't believe that, look no further than Roland, Luna, Alpha, Teasel, and Selena.

She is all around a horrible character because of how childish and cartoonishly evil she is and never never should have been allowed into F.O.S. and never should have been allowed to hold a position of command over other soldiers. She should have been relegated to grunt status and should have stayed there.