  In this paper we excavate, for the first time, the most general class of
conformal Killing vectors, that lies in the two dimensional subspace described
by the null and radial co-ordinates, that are admitted by the generalised
Vaidya geometry. Subsequently we find the most general class of generalised
Vaidya mass functions that give rise to such conformal symmetry. From our
analysis it is clear that why some well known subclasses of generalised Vaidya
geometry, like pure Vaidya or charged Vaidya solutions, admit only homothetic
Killing vectors but no proper conformal Killing vectors with non constant
conformal factors. We also study the gravitational collapse of generalised
Vaidya spacetimes that posses proper conformal symmetry to show that if the
central singularity is naked then in the vicinity of the central singularity
the spacetime becomes almost self similar. This study definitely sheds new
light on the geometrical properties of generalised Vaidya spacetimes.
