The dash is a friction fit and doesn't really "tighten up."

The whole cluster may be loose and could simply be pushed in more. Sometimes the heavy felt insulation comes unglued and makes seating it snugly difficult. You might have success by removing the knee bolster and 'guiding' it from underneath (I recommend wearing goggles if you're under the dash looking up - lots of crap up there) while someone gently pushes from the front. That may seat the whole unit.

Or, if someone pulled the clear lense while trying to remove it, it's possible the lense has come unglued/cracked away from the black housing and that's what's vibrating. If that's the case, then you can remove the whole cluster and glue the loose section back to the case. Don't be tempted to completely remove and re-glue the clear lense, just some super glue on the loose part.

If you remove the cluster and this is your first time (assumption based on post count) then search for some threads about the speedo cable and the oil line. You don't want to break it, but you want it to go back without leaking.