Ubisoft have now told us what went wrong with Vegas 2, though sadly they say they're unable to support workarounds that might help people continue playing.
One, it's still a great game! Two, many players were under the belief that Ubisoft had already closed its servers, because it's been unplayable since December. When Ubisoft announced this month that they would close online services for Rainbow Six Vegas 2 (and a few other tactical shooters), it was a surprise for two reasons.