Thinking about time loop it could have been that the Vorlons did NOT know that telepathic abilities can block the shadow's vessels.
When Valen appeared he brought with him voluble information from the future, thus giving the "forces of light" the advantage they needed to end the war.
Remember that the B5 universe is a looped one, the B4 traveling back in time, the artifact that changes Dellen and Sinclair, all of these occur again and again in that time loop. The war 1000 years ago could not have been won without B4 and Valen information about the shadows, information he received from the Minbari and Vorlons who survived and won the last war and thus able to help the war this time around (and give all that is needed to the war 1000 years ago that helped in this war that helped in that war etc.)
When Valen appeared he brought with him voluble information from the future, thus giving the "forces of light" the advantage they needed to end the war.
Remember that the B5 universe is a looped one, the B4 traveling back in time, the artifact that changes Dellen and Sinclair, all of these occur again and again in that time loop. The war 1000 years ago could not have been won without B4 and Valen information about the shadows, information he received from the Minbari and Vorlons who survived and won the last war and thus able to help the war this time around (and give all that is needed to the war 1000 years ago that helped in this war that helped in that war etc.)