xna - 3d world vertex translation to go to 2d screen coords

My technical english is a little rusty so to avoid misunderstands please be patient with me :)
I will try to be brief and clear

Situation:
- I have a 2d sprite character on the screen
- I've just start learning how to draw 3d primitives and work with the cameras
- I'm working on a 2d isometric environment
- I made a 3d isometric triangle which I want to center in character
- I'm trying to do something similar to a flashlight

Problem:
- The triangle is not centered on character (probably because of the scales between 2d and 3d workspace)
-the triangle speed does not match the character speed.

Code:

player vision

        playerVision = new VertexPositionColor[3];

        playerVision[0].Position = new Vector3(-15f, -30f, 0f);
        playerVision[0].Color = Color.Transparent;
        playerVision[1].Position = new Vector3(0f, 0f, 0f);

        playerVision[1].Color = new Color(70, 102, 25, 20);
        playerVision[2].Position = new Vector3(15f, -30f, 0f);
        playerVision[2].Color = Color.Transparent;

-

private void SetUpCamera()
{
    // isometric angle is 35º , for 50y = -107z
    viewMatrix = Matrix.CreateLookAt(new Vector3(0, -107, 50), new Vector3(0, 0, 0), new Vector3(0, 1, 0));


    // TODO: use viewport for whole window or just the draw area ?
    projectionMatrix = Matrix.CreatePerspectiveFieldOfView(MathHelper.PiOver4, GameInterface.vpWholeWindow.AspectRatio, 1.0f, 300.0f);
}


private void drawPlayerVision(GameTime gameTime, SpriteBatch spriteBatch, Vector2 playerDrawPos)
{
    // 50 is related with the camera position in the axis
    // TODO: this probably is incorrect since it should be a module of the distance? btw points
    Vector2 playerPos = playerDrawPos / 50;


    effect.CurrentTechnique = effect.Techniques["ColoredNoShading"];

    effect.Parameters["xView"].SetValue(viewMatrix);
    effect.Parameters["xProjection"].SetValue(projectionMatrix);

    // rotating like a radar
    Matrix worldMatrix = Matrix.CreateRotationZ(3 * visionAngle);

    // moving to the character

    worldMatrix *= Matrix.CreateTranslation(new Vector3(playerPos.X, playerPos.Y, 0));            

    effect.Parameters["xWorld"].SetValue(worldMatrix);
    //effect.Parameters["xWorld"].SetValue(Matrix.Identity);

    foreach (EffectPass pass in effect.CurrentTechnique.Passes)
    {
         pass.Apply();
         Game.GraphicsDevice.DrawUserPrimitives(PrimitiveType.TriangleList, playerVision, 0, 1, VertexPositionColor.VertexDeclaration);
    }

}

if you need more details please ask me I've been trying to figure out all by myself so its a little hard for me :)

Thank you for your assistance

Answer

I found that the issue was related with the usage of viewports. When defining viewports for 3d we need to define 2 extra props minDepth and maxDepth.

If we forget to define these two... the viewport is still valid for 2d but using unproject in 3d will return the NaN issue