I recently moved some virtual hosts from an older Ubuntu 12.10 to the newer 14.04. The problem I had was that I could not get the automatic user selection in apache to work for the mpm-itk modules. The module can be installed but it does not appear to configure properly.
To fix it I had to add the following line to /etc/apache2/apache2.conf
LoadModule mpm_itk_module /usr/lib/apache2/modules/mpm_itk.so
After doing that the virtual host definitions including the directive
AssignUserId <user> <group>
Started working as expected again. I thought it was weird it did not work out of the box, people have reported various issues with mpm_itk under Ubuntu 14.04 but I never found a solution to my particular problem so I hope this helps someone else to sort it out.