1) Open up Java Preferences, located in your Applications:Utilities:Java: folder.
2) In the bottom-half of the window in the general tab, labeled "Java application versions," drag the second line, "Java SE 6 64-bit," to the top. It should end up looking like this:
3) Launch the client
4) Switch the lines back again, or press the "Restore Defaults," for applications that are expecting the JVMs to be found in their default order (e.g. Vuze