I assume that it was planned long ago. ITV doesn't trust the housemates to carry the season. We've seen this on all of their series with the constant tasks and twists. And the finale getting broken down to stages might be something that we'll see on more Big Brothers around the world.
Celebrity Big Brother in Germany has been doing it for a few years now. They have two competitions on finale night and the losers of the competition face the public vote. Two housemates are evicted that way, then the remaining three people face the final vote to win.
Italy hasn't had any competitions, but the semifinals and finals of their previous season also had different stages. They had 11 housemates for the semifinals and some of them have already won a spot in the finale. During the semifinals they had three different votes for housemates to reach the finale, evicting four people in that process. Who was up for the vote, determined on a random draw and then the nominees picking another person to join them.
Eight housemates made it to finale night. The three housemates didn't win their ticket for the finale were up for the final eviction on finale night. Two were evicted, thus leaving six in the house. They were split into two groups of 3 with their separate votes. The one with the fewest votes from each group had to leave the house before the final 4 faced the final eviction vote.
Unlike the UK, both Germany and Italy have done that over one night (or two nights, if we include Italy's semifinals), but the live shows in these countries run for 3-4 hours. But I could still see that (compeition/random draw/random eviction lists) being the inspiration for the Vault on BBUK. However, neither of these versions fooled the audience into thinking that the votes are the votes to win (until it came down to final 3/final 4) and all votings started after the audience was informed who was up for the public vote.
Germany and Italy have too much time to fill for the final show. And since ITV has sadly no faith in their housemates carrying the series, they think they have too much time to fill for the final week.