The official information specifies that with every release of VectorWorks, the file format changes, but it doesn't mean there is no possibility to open the files. On the other hand, yes, you can use VectorWorks 2012 to open VectorWorks 11.5 files. For more information I recommend visiting the official help provided by VectorWorks which explains exactly what versions are supported by VectorWorks.
The official information specifies that with every release of VectorWorks, the file format changes, but it doesn't mean there is no possibility to open the files. On the other hand, yes, you can use VectorWorks 2012 to open VectorWorks 11.5 files. For more information I recommend visiting the official help provided by VectorWorks which explains exactly what versions are supported by VectorWorks.