As Bruce says, the triangle panel is attached to the nose leg, behind the front nose gear door and fills the triangular gap between the front gear door and the rear door.
The single seaters don’t have this, I wonder if it makes up the difference in gear bay length, but can’t be attached to the front door as would be too close to the ground if the nose leg oleo bottoms out?

This pic shows the layout,

Edit, forum problems not enabling pic, so here is a link to a pic of the door layout, http://www.williammaloney.com/Aviation/QuonsetAirMuseum/DeHavillandVampire/images/05VampireNoseGear.jpg