Player.Pos and Monster.Pos are 3D vectors (TD3DXVector3) and describe world position of player and monster. They are composed of x,y,z variables you use currently.

Player.Dir is also TD3DXVector3 describing orientation of the player (or monster). It can be made from nrotatey variable you have

Player.Dir := MakeTD3DXVector3( cos(nrotatey), 0, sin(nrotatey));

If you store nrotatey in angles dont forget to change it to radians before you pass it to sin and cos functions.

vDir is also a TD3DXVector3 but in my example it is replaced by Player.Dir.

v is also a TD3DXVector3. So in the line
v := Monster.Pos - Player.Pos;
"-" means vector subtraction.

d is a scalar value (single or double).

I am sorry for the confusion. I hope it is more clear now.

I just noticed that it is already anwsered. I am so sloooow typer .