Do I need an additional lens?

I have a Nikon 18-200mm lens. I'm gonna do some very important portrait shots and quality is the number one priority. Would I be better off with a prime F/1.8G 50mm lens or use the one I already have?

You will get So much better portrait quality with a f1.8 50mm lens, not least because the wide aperture will help you put the background nicely out of focus.

For portraits you want the Nikon AF-S 50mm f1.8G.

Get the prime it is a very useful lens and about prime vs zoom for portraits i would choose a prime it offers good quality over zooms

Mmm portraits

The 18-200 has the range to do portraits but due to what it is. It will not be the best choice.

The 18-200 and really any long range zoom tends to have weaknesses. Points where it is simply meeting the limitations of physics and money. Sharpness might be lacking. You might have dark corners or even purple highlights. In short a 18-200 is nice to walk around with but if you really want specific something there are better lenses.

For instance portraits. One trick is to put your zoomlens to 50mm and not to move it from there. Then use a friend to do a portrait shot. You probably notice that at F3.5/F4 it isn't as nice as you hoped. Also shooting wide open even on that… Just doesn't blows out the background into a nice soft bokeh.

At least not as good as an actual 50mm. Even the cheapy plastic F1.8 already will do wonders compared to your current lens for portraits. Shoot it at around F2.5 and make sure the eyes are sharp and you get a nice soft Out Of Focus background with sharp where needs to be images.

So yeah I suggest you buy the 50mm. Even the humble F1.8 already is a big difference. Remember the eye has to be sharp. Not to mention you'll find that your camera suddenly focuses quicker… [cameras love lenses that are F2.8 or quicker, more light means quicker focus]

And best for the money… Again the plastic fantastic 50mm F1.8 is cheap. But still has good glass in it so go for it

If light is no problem, the 18-200mm will do. Stay between 40mm and 80mm preferably around 50mm. You won't need the 50mm f/1.8 unless you need to blur backgrounds more easily. If you can make the subject closer to your and far, far away from the background then the background will blur just the same with your existing lens.

But! If you have cash to spare then go ahead and get the 50mm f/1.8.It will make things a lot easier. If you have a body that has no AF motor, make sure you get the AF-S or AF-I version of the lens to preserve autofocus compatibility. With closeups at f/1.8 at less than a meter away, you would be better off using manual focus anyway (more accurate) but still it would be nice to have AF when you need it for other shooting scenarios (like action).