Vegas didn't go wrong. This is what Bees is saying above. Vegas isn't in the business of accurately predicting game spreads. Vegas is in the business of making money. They set their spreads, not based on what they actually believe will happen, but on what they believe will draw approximately even betting on either side of the bet. It's all about predicting public perception.
And what exactly do we know?
1. That we have no idea how the top 2 WRs will be replaced.
2. That the LBs will be largely untested.
3. That the same less-than-spectacular RBs are back.
4. We really don't know if the OL is any better.
5. We don't know if DL depth is any better.
Vegas evaluation, AT THIS TIME WITHOUT FURTHER EVIDENCE, is spot on. No answers to these questions probably until game 3 of the season.