Actually you would be unable to purchase the dining plan if you went with my strategy of purchasing your vacation needs separately.
I usually "shop" for the best discount that I can find on tickets and accommodations since these are the two major expenses for my family. However, we are only a family of 3 and meal costs would be more of a factor for larger families.
What I would suggest is pricing out your tickets, room and meals separately. Use the menus at allearsnet.com to estimate your costs. Then, price out a package that includes the dining plan. Sometimes the price difference will be within $200, for example, and for some people the convenience of the package is worth the extra expense. Remember that there is also stricter cancelation policies with packages.
In order to add the dining plan to your vacation you must either be a Disney Vacation Club Member staying at a Disney Vacation Club resort on points, or a non-DVC member booking a Disney Magic Your Way package for accommodations along with at least one day of park admission.
For more details, look
here.
Remember to take into account what is best for your family. Don't purchase a package that has items that you will not use. The Dining Plan is a great way to save on meal costs but make sure that you are not paying more for the package than if you were to book your room, tickets and meals separately. If you don't mind doing a little research, you'll soon find whether a package or separate vacation components are best for your family. You want to try to save the most on what will be the largest expense for your family. If it is your hotel room, do not pay rack rate. If it is your meal costs, consider a package with the dining plan.
I hope that I've helped clear up some of your confusion.