Since you say you are not so lucky using patterns...........
I just wanted to say do not buy a vintage pattern. You will most likely find that there are no seam allowances, different closure methods in some cases, and probably not enough instructions. If you can't find a tutorial or a second hand dress then you should try to find a modern pattern with a vintage style.
When I first started using patterns they were confusing to me too, but I found a trick that worked for me.
The pieces are the easiest part, cut them out to your size and lay them down and cut them out. There will be a picture of how to lay them down.
The instructions.......That was what got me. Some of them I didn't understand, and turning the pages got confusing so I kept losing my place. I started re-writing the instructions on normal paper as a checklist. I could cross off the steps I had done.
If I didn't understand how something was supposed to fit I would stop sewing and start using safety pins until I figured it out.
If you do end up using a pattern and start to have problems please post back here and ask what to do. Or PM me, or email me at firstname.lastname@example.org
The play sounds fun by the way! I always wanted to do theatrical costuming but we don't have stuff like that here (Saskatoon, SK)