I assume that by "vertical," slawecki means a tasting of multiple ages or vintages for a single brand. For example, at Taylor Flagdate, you can taste the 40-, 30-, 20-, and 10-year tawnys in succession. This, of course, accentuates the differences due to the age since the formula is the same otherwise. You could also do this by tasting a series of vintages, which would be even better if you could pick out the especially memorable ones.

I assume that "single" means tasting a number of brands that are the same age or vintage, for example tasting the 1977 vintage of a number of different producers at the same time to understand the differences in brand.