EDIT: I am updating this answer because as of 18 Jan 2012 Google has just announced that:
The standalone Contacts service now has the same interface and
features that are currently available in Contacts inside of Gmail.
Make sure you use the contacts view available from GMail and NOT the one at contacts.google.com which has not been updated by Google in a very long time.
The rest of these instructions are still correct:
- in Gmail contacts view select the entry you want
- Click on "More actions" and then "Export..."
- Leave the default for "Which contacts do you want to export?" as "Selected contacts (1)"
- Change "Which export format?" to "vCard format"