What else? plist file? and?. . . ?

That's quite a rats nest you've gotten yourself into :)

First off, I'l say I don't think there are many absolutes here - no single reason why this is happening, nor any single action you can take that will resolve it. It's going to be some level of trial and error.

To that end, copying .plist files isn't likely to do anything - there is no code in the .plist, just (typically) preference settings, and they shouldn't be able to affect this.

Second, given the age of the software, it's likely that the app is self-contained... that is, everything you need is in the .app file. Modern apps tend to consist of many discrete files, libraries, plug-ins, extensions, etc. but back in 2008 the norm was more towards a single monolithic .app file. Therefore it's likely that copying the 13.0.1.app file is all there is to do - there are no other files that contain executable code that could (should?) affect this.

There' a chance, though, that copying the app doesn't fix the problem.

Double-clicking a document in the Finder is the responsibility of the OS and not the target app. The Finder has to lookup the document type, find what kind of application can handle it, and initiate the application (launch it, bring it frontmost, etc.) and hand it the document.

It isn't clear from your description where the problem lies - is the error message you get coming from the Finder ("No known application for this document type"?), or from VectorWorks (the app opens correctly, but VectorWorks says it can't open the document? The difference is key.

If it's the Finder that's throwing the error, then it's the OS that's lost track of which application can open the CAD file. The OS keeps a map of file types and applications, so it knows that, for example "Pages" can open ".pages" ".docx" and ".txt" files, Numbers can open ".numbers", ".xlsx" and ".csv" files, and so on. If this map has gotten corrupted, then copying the application file isn't going to make a difference since the problem occurs before the OS knows what application to use

(note, contrary to what I said above, each application uses a .plist that contains data on which documents it can open, and the OS reads this when installing the app, but it's embedded within the application and not a separate standalone file that users normally access).

If the OS is properly launching VectorWorks when you double-click a document, then the OS<->Document Type map is working, and it's something else that's at fault.

Knowing where it breaks is going to be key to finding a solution.

eXtended Markup Language (XML) is just a file format type the structures file contents in a predictable and easily parsed way. plist files are a particular sort of XML-formatted file - and many CAD file formats are also XML under the hood.

Best guess is that the older version of your application had a subtle bug in it's XML parser that only triggered under certain conditions - like perhaps if a CAD file created in a previous version got opened in 13.0.0. It's even possible that 13.0.1 specifically fixed that bug (among others).