Not to take away anything that's been said but any manouver with a vertical component, either up or down (i.e. yo-yo's, spiral climbs, defensive spirals, etc.) qualify as "keeping things in the vertical". That said, in practicality, when most people discuss this it's keeping things in the vertical going up, where certain planes outperform others.
The vertical portions of the fight may be limited by zoom climbing ability (as with the F4U's, Jugs, 190's, etc.), or may be continuous based on raw horsepower to weight ratios (as with the 109's, P-38's, Spits, etc.). The manouvers available to you will be dictated by this, by your E state and by your enemy's E state.
It does you little good to go vertical if your enemy can still close on you until you stall at the top. Often, oblique turns as a vertical component are as valuable, if not more so than a pure vertical manouver.
The point I'm trying to make is that, while with certain planes keeping the fight in the vertical may be generally good advice, it's still entirely situation dependent. There are so many variables in any engagement that specific manouvers may have no relevence.
I'd suggest learning and practicing vertical manouvers, keeping them in your bag of tools and pulling them out as you see an opportunity to use them. It's more a matter of learning acm and what's valuable at what times in what situations.