How long is Revan allowed to use the SF for?
It takes time for it to grow his power.
Also Malak knew about Vitiate(met him), when he mused that Revan would of became invincible with the SF, so that indicates SF Revan>>Novel Vitiate
Darth Revan was clearly aware of a galactic threat in the Unknown Regions during the Jedi Civil War. It's likely though that the specific memory of Vitiate was buried.
As for Malak, he never seemed to exhibit care over Vitiate's Sith Empire one way or another, so he might have near-completely purged it from his mind.
__________________ "There is only Revan. Only he can shape this galaxy as it is meant to be shaped."