Sunbird now have .vcs calendar support supplied by an add-on: https://addons.mozilla.org/en-US/sunbird/addon/4721. The really nice thing about this add-on is that it supports exporting of .vcs calendars. So if you're current calendar app doesn't support .vcs export, but you need it to. You can export from your calendar into Sunbird, and then export from Sunbird. A little cumbersome to be sure. But if you're stuck for a free solution, it definitely works.