The Wikimedia production application servers are being migrated from Ubuntu 14.04 Trusty to Debian Jessie. Our vagrant setup should follow that lead in order to maintain maximum compatibility with testing production changes.
There are several big things that will need to be dealt with, but probably the biggest is that Jessie uses systemd and removes support for upstart. Roles that include upstart service management will need to be updated to use systemd units instead.
As we did with the prior precise to trusty migration we should remember to tag the repo with a "trusty-compat" tag to allow pinning to the older version of code for VMs that for whatever reason can't be upgraded.