Well ???
I don’t think it’s actually defined anywhere :) I think 3 makes some sense, although 1 works if you’re trying to be funny. 
I was trying to be “Vertical” I got 3 yrs of Bigfoot Barleywine…gonna wait one more yr. for
the main tasting, but I have an extra 2010 and an extra 2011 to go now…
You’re not trying to be 1vertical? 
I think 2 is still vertical enough, especially if that’s all you have.
vertical enough, huh? can you be ‘more’ vertical?
It depends on what the definition of “is” is.
You only need two points to describe a line in 3-space. Of course, you have to define your axes, or “vertical” is a meaningless qualifier.
does that mean you can have a horizontal qualifier? 
I would think any beer you drink would be considered a Verticle tasting. :-\
ROLF ;D da weaze!
Not trying to be a smartass, but at least two.
The whole point of a vertical tasting is that you’re trying to get a sense of how the same beer recipe ages over time. For that, you need two different batches, or one batch aged under different conditions (e.g., one kept at 32 F for a month, one kept at room temperature).
For big beers which develop slowly, it’s more useful and more fun to have a flight of at least 3 to 5, though.
Thank you thomas, I can see that the more the merrier applies here…
It all depends upon the beer…I kinda like 10 for beers that might stand a chance at holding up that long…at least 4 IMO…or if the beer might be heading downhill, 3…
I could see that perhaps my Avery Beast could make it that long, but sheesh 10 yr old beer. I think beer
is made to be consumed. Ah I got to wait 2 more for that Avery Beast to get ripe…then prolly gonna have
a taste just because. That is not a vertical taste either…just a reserved special bottle.
At GABF a few years ago, Alaska Brewing was doing vertical tastings of their Smoked Porter. They had 4 different vintages each session. I was there on the Thursday night session and enjoyed a 10 year old sample compared with more recent ones. It was the highlight of the evening for me. The oldest example was pretty vinous (winey), but it was cool to taste the progression.
Three points makes a trend but a steady trend is more. I think 5-6 would make a nice vertical.