Try this. It's not exactly a maths way, but...
Create a pivot at 0,0,0
Create another pivot at 0,0,0 and parent it to the one before.
Position the pivot at 0,0,1 (which i assume is the correct vector for pitch and yaw, if not, adjust accordingly).
Then... rotate the pivot at 0,0,0 to the pitch and yaw values. Then read the global position of the second pivot. The X,Y and Z values will be your vectors.
No doubt someone will come along with a maths way, but that will work for you until then.
Create a pivot at 0,0,0
Create another pivot at 0,0,0 and parent it to the one before.
Position the pivot at 0,0,1 (which i assume is the correct vector for pitch and yaw, if not, adjust accordingly).
Then... rotate the pivot at 0,0,0 to the pitch and yaw values. Then read the global position of the second pivot. The X,Y and Z values will be your vectors.
No doubt someone will come along with a maths way, but that will work for you until then.