b) For me the period of concern here isn't once all the vulnerable are vaccinated, it's the period, likely to be a good few months, where some are and some aren't. If vaccinated over 70s stop following the law because they feel personally safe then if case numbers rise significantly, whether it's as a result of them transmitting it or (which seems more likely to me) because this causes a general collapse in compliance, the end result is that groups like the at-risk under 65s, or 60-65 year olds, who are a lot lower down the vaccination list, will be at significantly increased risk.
Firstly, the chances are high that the vaccine will stop or massively reduce transmission in those who've had it, so 'vaccinated oldies' are unlikely to be a huge source of risk to the unvaccinated.
Secondly, if we're talking about attitudes to mixing after vaccination - I think the unvaccinated young and less vulnerable are likely to be a far bigger risk. I have heard a lot about how once the vulnerable have been vaccinated everything should get back to normal ASAP.
I agree that it's the middle aged/younger old people (who are far down the vaccine list but still a bit vulnerable) who will be most at risk during the time when we're only halfway through vaccinations. I'm not at all convinced that the risk to those people is going to come from reckless vaccinated people more than from the reckless unvaccinated people who are keen to get back to normal.
At least in theory, middle-aged people are going to be at risk from both those groups - but given that the vaccine probably will reduce transmission a lot, probably we'll be a lot more at risk - by numbers - from younger unvaccinated people. We'll be relying on both groups to modify their behaviour to reduce a risk they perceive as only affecting others, not themselves.
I don't believe that age is going to be the thing that determines how people choose to behave. I think personality, education, individual character, family attitudes that cross generations and so on will make more difference to people's behaviour than their age.