The vpim is not very active, icalendar is a bit more active. The idea is that both of these ruby libs lack proper iCalendar timezone support (I think the icalendar has just added TZ support but I didn't test it yet) and proper recurrence handling (from what I've seen icalendar doesn't do any recurrence)
Libical is by far the most used icalendar parser out there.
HTH Cosmin