Most famous "vaporware" computer: Xerox
Alto.
I must vigorously disagree with this one. The Xerox Alto was only
offered to paying customers years after it was developed, and I've heard
no reports of unreasonably delivery schedules. In fact, until recently
I was of the impression that it had never been offered for sale at all,
but apparently it was offered to very select customers, such as Atlantic
Richfield and the US Congress.
It's only vaporware if the manufacturer offers/advertises it but does not
actually have it available for sale.
If you want to nominate a Xerox computer system for the category of
longest schedule slip, it would be the Star, which was several years late
but eventually did ship using the Xerox 8010 (Dandelion) hardware.