The risk is going to be longer, assuming our current outlook in California of wanting to keep everyone "safe". Firstly, this assumes a "magic" vaccine which is 98% efficient and keeps people completely immune even if it's for a short time). The reality is the initial vaccines are going to be much less efficient, particularly with those with compromised immune system like the elderly (who BTW, are the ones most vulnerable anyways). Remember despite having a vaccine H1N1 is still with us and the vaccine is not fully efficient.
Then remember they aren't going to have enough for everyone. They'll start with health care workers, essential workers, the elderly, but those looking to get back into offices waiting on a vaccine are going to be disappointed (particularly as it seems now multiple doses of the vaccine are required). Also note the pediatric vaccine has not entered phase 3 testing. We are looking at summer 2021 at the earliest for kids to get vaccinated.
Then it assumes that people are actually going to take it. Surveys have shown large numbers are skeptical of the vaccine, particularly when it comes to children, and many will decline to be in the initial rollout of the vaccine (given, rightly or wrong, the perception that it's been rushed and long term effects not studied). The government may need to force people to vaccinate (some states like Virginia and Hawaii have indicated they will force people).
This is why health experts are saying Americans must be prepared to wear masks for years to come, even after they've been vaccinated. That's why the governor's plan has no "green level" return to normal. Further, the governors standard's don't care about how sick people get, hospitalizations or deaths. Even if the vaccine reduces the severity of the illness in a lot of people, as long as it's circulating and some group of the population remains vulnerable, the governor's plan contemplates restrictions-- in some low population counties having a few positive cases is enough to send them back up into the higher levels of restriction. The thing could mutate to something as ubiquitous as the common cold, but if a handful of people still die from it California's standards still contemplate years of restrictions.