The problem here is, Flash thinks much faster then Vulcan does, so the Flash will definetely get the first hit,(Through a Speed Blitz, or by Causing Gabriels molecules to vibrate very quickly.) However during that first attack Gabriel takes this with ease. He possesses Darwins power of Advanced Evolution meaning he could just adapt to the Flash's powers/attacks(He can adapt to the Speed Blitz and the Vibration. He can also prevent the Flash from stealing his speed by adapting) making the Flash useless.
Then Gabriel can pretty much negate and duplicate the Flash's superspeed, making himself pretty powerful. And could either start manipulating Chaos and Mystical Energy to Reality Warp and blink the Flash out of existence or he could drain all the energy out of the Flash thus killing him(The Flash is still a living being and possesses an energy life force) or he could just kill him with his hands.
Since the Flash would be the hero in this story he should have at least a small chance of winning due to some miracle. Therefore Vulcan takes this 99.999% of the time.(Don't worry this whole Omega PLUS mutant thing will most probably change in the future, as Vulcan WILL be depowered ALOT to keep the X-Men on top.)