It might be difficult to find an official download for VectorWorks 10.5 as it is an older version and likely no longer supported by the developers. You should check with VectorWorks' official website or contact their support team to see if they can provide you with a download option. Alternatively, consider upgrading to a newer version of the software, but make sure it can open or convert your old drawings.
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.
all my old drawings are made in 10.5
It might be difficult to find an official download for VectorWorks 10.5 as it is an older version and likely no longer supported by the developers. You should check with VectorWorks' official website or contact their support team to see if they can provide you with a download option. Alternatively, consider upgrading to a newer version of the software, but make sure it can open or convert your old drawings.
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.