@jakez I was able to get VW2018 installed by following the instructions posted above. In case it's not clear, you have to extract the installer from the zip folder you downloaded, then find the file named "Vectorworks 20XX Installer" and extract that, as well. There are two levels of extraction.
For the record, the computer I installed on is a Windows 8 PC, and I used 7-Zip for the extractions.