I dont know about Gregg, but in Pitman shorthand, the position of the consonant's outline (as the symbol is called) relative to the line dictates its associated vowel. Above the the line is "ah", on the line is "eh", below is "ee", and so forth. You can adorn the whole outline with dots and/or dashes for vowel sounds on all consonant outlines, especially since its written cursively (ie a whole word is writtent together). However, in practice, you get what the word is from context and experience. My father who was a stenographer made me learn it when i was a kid and to this day I remember the outlines of words quite easily, even though I've never used it professionally.

Edit: A more appropriate question to the part of the article you referred to would be: How do you differentiate between "cot" and "got", for example, because it would be the same outline for c(ie k) and g. The official answer is that you wrote one thinner and the other thicker (which is possible when you use a pencil, which was what was used to write pitman shorthand, anyway). The practical answer is that you get it from context - not all sentences could have both those words equally viable.