Extracting the data from the delta file is very difficult, due to the way the data is stored (grain directory, grain tables, ...). What I could think of is to create an additional virtual disk on a helper VM with the same virtual disk controller, same partitioning and same formatting as the previously deleted virtual disk. Then create a snapshot and replace the delta file for this new virtual disk with the 18GB file you still have (by copying and renaming it). Make sure you replace the correct delta file for the additional virtual disk! Before you power on the VM, create another snapshot to ensure the 18GB delta file does not get modified and to be able to revert in case something doesn't work as expected.
André