If you have access to the server's configuration, or you know a bit of scripting, you can create a default mime type for a certain suffix.
So (for example) a file called card.vc could be mapped to a bogus type of 'application/x-custom-vcard' so any browser that requests it will bypass most of its inbuilt handlers and, in most instances, ask you what you'd like to do with that file.
Or you could change things at server level and create a script that forces a "Content-Disposition" header with the content of "attachment; filename= yourfilegoeshere.vc"
What kind of setup do you have? More information, please!