The ability to sort projects by categories is fairly limited in iMovie 10.
You can go to the projects browser view (where your projects are displayed as icons) and click on View/Sort By. You will be given options to sort by date, name, or duration. Perhaps if you use alphabetized prefixes they might group together by the designated prefix when sorting by name. You'd need to experiment.

There's also a search box at the top of the projects browser screen where you can search by title. For example, if you titled all of your vacation videos with the prefix Vacation - (followed by title name) then when you searched for Vacation in the search box all of your Vacation projects should show up. Same with Birthdays, Weddings, etc. The prefix word would be searched.
Also, since you can create as many libraries as you want, you can create them to hold projects in different categories, such as Birthday, Vacation, Weddings, Holidays, Europe Trip, etc. So if you wanted to view all of your Vacation projects, you would open the library titled Vacations.
-- Rich