Opening the borders will not be binary. It's not premature to ask the question. Vaccines will definitely be part of the phased entry requirements change. It will likely also be one of the longest-standing ones - it will remain long after a negative PCR test is required for example. It will probably persist until the pandemic is pretty much gone globally (through mass-vaccinations even in developing countries). The timeline for that is probably late 2022/early 2023.

So, there is an 18+ month window where entry may be allowed into (as an example) Europe with a vaccine being a key requirement. My guess is that by mid-2021, a vaccine + negative PCR will allow entry for most purposes (possibly not straight leisure trips unless things are fully under control) but by late 2021 that may shift to a vaccine only if prevalence drops sufficiently low in both regions.