If the situation was the same as it is now - (ie including with vaccines of unknown efficacy being "imminent") then why would you do anything different that we're doing now? If by the end of next year it was known that a vaccine wasn't going to work - then you make decisions at that time based on the good evidence you've got then - not based on what we know now.
Discussions around the border need to be happening now and not just put in the too hard basket. A blanket ban on travel to NZ isn't sustainable even medium term, and in particular exclusions on people coming here as skilled workers (whether long term or short term) and the massive implications for the literally thousands and thousands of FIFO workers based in NZ who are currently stuck in limbo.
I think a cost benefit analysis needs to be done based on risk. There is potentially going to be a high cost as soon as community transmission is detected again. Another lockdown will cost the NZ community and businesses money and require the government to again provide more handouts. If the first lockdown has a cost of 50 billion we have to look at what revenue we get by letting more people in and using private contractors potentially increasing the risk of community transmission due to human error or holes in processes.