IIS sends an http 404 if user requested a file type that IIS does not have a mime type set. Do the following to serve vsix files.
Add the following to
system.webserver of root web.config
<staticContent> <mimeMap fileExtension=".vsix" mimeType="application/vsix" /> </staticContent>
Or go to mime types in IIS manager (inetmgr) and add the .vsix extension with the mimetype as application/vsix