I would say you should ask Apple this question.
Hmm...really I wouldn't even call that "vertical" text. It's only rotated, (meaning it surprises me that any search misses it just for that reason).
Of course I can't account for Spotlight's shortcomings. If you don't come up with any other solution, I'd suggest trying things like setting the number on the page "invisibly." Methods that might work (for the purpose of search) include text with no fill color or perhaps masked behind another object; even if it's just a white-filled rectangle.
Meanwhile I did some tests.
It appears rotated text IS spotlight-searchable, BUT words are seperated in parts.
For example: the text "ROTATED"
At 30°, spotlight finds the file until i type the "E" character.
30°: ROTAT - ED
60°: ROTA - TED
90°: RO - TA - TE - D
It seems to split the words, and spotlight sees the parts as different words.
While acrobat, mac preview etc. can search any combination characters, spotlight needs the first character of the word (it would never find "OME WOR" when "SOME WORDS" is in the file).
I tried using tags, hyperlinks / bookmarks, script labels etc, many different PDF export options (PDF tags, invisble objects, PDF versions,…).
I also played with some InDesign text controls that could cause the text splitting, like kerning/tracking settings, opentype features, hyphenation turned off...
Also with mirroring (mirrored text is indexed by the way), vertical text in a non-rotated table.
None of the above made the file spotlight-searchable.
Opening the PDF in Illustrator shows the the text as 1 part, so I don't know why spotlight sees the words as seperate parts.
John's invisible text-option looks like the solution to go with.
Perhaps the "linked content" feature in can handle this, to make sure the visible and invisble text is always synced. (just switched from CS5.5, and didn't test that feature yet).