Store the random Vector3 as a variable and lerp to it.
var target : Vector3
func _ready():
while true:
target = Vector3(randf()*11+1, randf()*11+1, randf()*11+1)
yield(get_tree().create_timer(0.5),"timeout")
func _process(delta):
$light.translation = lerp($light.translation, target, delta*2)