Yes.  By default, all content you download to your Library using Vuze is in turn made available for others to download through "seeding".  However, you can change this behavior for any or all of the files in your Library, as follows:

• Select "My Library" in the left navigation area of the Vuze application.   All the content in your library will be displayed there.  Note that there are two ways to view your Library.  The more detailed view is selected using the toggle switch at the top of the page or the "Advanced List" entry in the View menu.
• For each piece of content, it will either say "seeding", "queued", or "stopped" in the Status column.  Any file listed as "seeding" or "queued" is currently available for seeding.
  
• Select a piece of content with your cursor.  If you want to turn seeding off for that content, click on the "Stop" button in the toolbar at the top of the page.   If you want to turn seeding back on, click on the "Start" button.

Note, if a file is being downloaded, you will also be "peering", which means you are making parts of that file available to others who are trying to download the same file.  Selecting "Stop" will stop the file both from uploading and from downloading; you cannot continue downloading the file and shut off uploading at the same time.