Vintage patterns can be in quite delicate condition so it's important to take the proper care so they don't get damaged.
Whenever, I first get a pattern I always take everything out and check to see if all of the pieces are there. It's also a great time to take a peak at the instructions and look at how the dress is constructed. Now, I personally don't like to put the pattern back in the envelope since I often further damage the envelope when stuffing the pieces again.
If any of the pieces are crumpled, flatten them out gently. I have heard that ironing on low heat with no steam can help particularly wrinkly pieces. To keep all of the pieces together, I put them in a clear plastic bag. If the pattern did not arrive in a bag, I use gallon sized zip lock bags. Comic book protective sleeves also work well.
When I go to use a pattern, I trace it onto sheets of tracing paper. You can get them in the drawing sections of craft stores. The pieces are too small for some things so I tape them together with scotch tape. This keeps you from damaging the pieces while cutting and is perfect if you need to make any pattern adjustments such as an FBA. If you have an unprinted pattern, take this opportunity to translated into printed directions.
I also photo copy the pattern cover to stick in a separate zip lock bag. It's also a good idea to photo copy the instructions, but since they are in a rather awkward size for home copiers, I usually skip this step unless the instructions are in particularly bad condition. (Shhh! Don't tell!)
Always store your vintage patterns in a cool, dry environment. I have some vintage pattern storage boxes that work great for vintage patterns. I also have one drawer of a plastic organizer full of patterns plus a second drawer for all of the traced copies. Comic book storage boxes are also a nice option.
It is also important to store your patterns away from other vintage and antique things like lace and linens since they can spread different kinds of molds to each other.