Vincent watched Tifa throw herself at Cloud with a vague sense of pity. Pity for Tifa, who couldn't see that Cloud had long ago set his heart on someone else, and even more for Cloud, who had given his heart away to someone who had never known what he was going through because of it.
Eventually, Tifa would push Cloud away, and then he would have lost his last reason to stay here. Her selfish love would never fill the hole left by Sephiroth, no matter how hard she wished it.
Vincent watched as Tifa ran headlong down the path to Cloud's demise, chasing an ephermal dream of ideal love. Life was tragedy, after all.