Omitted Short Vowels--Any Consistent Rules?

I have had a hobby interest in shorthand for years, and I'm currently studying Simplified.  I am fascinated by it, use it a lot, and study on nearly a daily basis.  I don't mind brief forms, have no major problems with the theory, and my reading is becoming admirably good.  There is one aspect of it, however, that is simply kicking my tail---I cannot get a handle on when I'm supposed to leave out short vowels (or even long vowels at times).  I study words in the dictionary and compare the standard outlines to what I am writing, and I simply can't find any consistent patterns.  Sometimes the vowel is omitted; sometimes it's not, and there's often no theory to tell you to leave the vowel out.  I thought it was just obscure vowels, but sometimes the vowel left out is the main vowel in the word. 
I see that single syllable words usually are written out in full, but poly-syllabic words often drop vowels, seemingly at random--but then they appear in odd places where the vowel used is quite obscure.  I can't going to go through and memorize an arbitrary, random outline for every polysyllabic word in the English language--there has GOT to be SOME pattern to all of this.  My questions are these:
1. Do people who use shorthand consistently drop the same vowels and usually write all of their words the same way (the way they appear in the dictionary)--particularly over time?
2. Does it really matter as long as I can write a clear outline, quickly?
3. Is there any place where all of this "extra" theory is explained in detail?  It seems that the thinking was that people would just pick it all up automatically as they read and practiced---but everybody doesn't pick things up by osmosis like that, obviously.
This is about the only aspect of shorthand that is really stumping me.  It often affects my speed, as I'm hesitating to try to recall if the stupid vowel should be left out or not.  I can give examples if needed---feel free to send me a message off the list if this gets too involved.  I really need some help with this.

(by Troy for group greggshorthand)