How can we make so that we can expand the VMFS to the maximum LUN size (2.79 TB).
Unfortunately you can't do this this way, at least not with VMFS3. With VMFS3 you are still bound to a maximum LUN isze of 2TB minus 512 Bytes. Since you already increased the LUN size, your options are now very limited. To increase a datastore prior to VMFS5, you need to use extents (separate LUNs). However I'm no friend of this and rather create multiple datastores than extending an existing one.
With the already increased LUN your options are to either recreate the LUNs with sizes <2TB (which requires backing up and restoring all the VMs) or to think of migrating to vSphere 5 and upgrade the datastore to VMFS5 once all hosts are upgraded.
André