That would depend his his level of situational awareness. Going vertical is my favored first move after a merge as well, as it's more efficient than a break turn to reverse and it CAN work, especially against an inexperienced opponent who has difficulty keeping track of your position. However if he knows to pick up your position immediately on merge he'll spot your vertical maneuver and will respond accordingly.
He may immelmann on you to force a second merge while you're coming over the top, enter a climbing spiral to deny you position on his 6, split-S under you to extend out to where you can't drop into an immediate guns solution, etc, etc.
A dogfight is one long series of Move/Counter-move. NEVER count on your first maneuver being the only one you need because if you don't have a second, third, or fourth planned out you lose the initiative the moment your opponent defeats the first.