That's not even a guarantee that we are able to create a vaccine that works and is safe. I hope that we find one, but there is always the possibility that we are not able to find one that fast.
I'm not a medical professional nor an epidemiologist.
But a crude rule of thumb is that the herd immunity threshold is equal to (1 - 1 / R0), with R0 the basic reproductive number (depending on the virus and how people are behaving, on average 1 persons infects R0 other people). Keep in mind that R0 is behaving kind of 'exponentially', so going from a R0 of 1 to 2 has huge consequences.
Here is the table.
R0 of 12-18 is for measles (as a reference).
R0 depends on the virus itself but also how society and people behave (washing hands, density of population, masks, public transportation, etc).
Estimating that R0 is quite difficult from what i understand, and you can see the effect of the error of measure on the thredshold needed.
Maybe R0 is between 2 and 6 ? If that's the case, then you need between 50% and 83% of the population immune to the virus to break new infections.