I haven't gotten any replies on this forum, so I knew my plan must be pretty out of whack. I went to a local plumbing supply store and talked to a master plumber who gave me a few hints, but wouldn't answer all my questions unless I was ready to drop a lot of money with him (ugh!). I've adjusted my plan according to the information I got, which was:
a) using a horizontal dry line is not acceptable, so my plan to vent the side of the stack won't work.
b) I can fully comply with code without the vent pipe in my diagram.
c) I only need one vent, which must be between the two highest fixtures.
d) he mentioned something about the junction of the vanity waste line to the shower waste line as being considered a wet vent, which allows a single vent to meet all code requirements for distances to the traps.
So... I took out two of my three vent lines out and kept only the one running from the vanity to the roof. This leaves the shower and stack to be wet vented. How does this look to you all now? Do you have any feedback on further revisions I can make? I'm going to send this diagram to the plumbing inspector and see if he/she will help me out too. Thanks for your feedback all!