The ongoing problem of Always A Bigger Fish meant Galian might be outstripped by his current opponent, but that Sephiroth too is inevitably going to cross paths with someone who can put him in his place. Maybe it'll even be Vincent, eventually.

But right now, Sephiroth has the advantage of sheer size and power, and as the beast's struggles lessen (though not weaken), he doesn't let go. Not yet. Letting go would likely mean a resumption of hostilities, and if he returned to his (stolen) treat still waiting on the hood of a car where it was left, his impression of this other-possible-chimera is that Vincent would attack again as soon as his back was turned.