Vinicius was the clear favorite at the end of club season. The players potentially in contention - Mbappé, Bellingham, Foden - needed a massive Euro to overtake him. It didn't happen. Rodri wasn't in the picture back then.
Rodri though he had a good tournament, didn't stand out amongst Spanish players. I didn't see the Man City version of him. He did win POTT, but Ruiz, Olmo, Yamal, Nico Williams would all have been worthy winners as well. They gave it to Rodri partly because he's by far Spain's best player on paper.
Now it's seemingly between Vinicius and Rodri. I don't think it should be. I think Vini's season coupled with his main challengers underperforming at the Euro make him the clear cut favorite.
Let's take a look at the Ballon d'Or criteria.
Team performances:
Draw.
They both won 2 major trophies this season.
Individual performances:
Vinicius.
They were both excellent in their respective leagues. Basically the best or second best players. However, Vini's UCL campaign is far superior to Rodri's Euro. He was Madrid's best player by some distance, scoring or assisting in every round, including in the final. And it goes beyond the stats. He was Madrid's main threat, and their only threat at times. The tactics were basically: let's give the ball to Vini and hope he creates something. In the last three games, it really was the Vinicius Jr. show. He was the unanimous POTT.
Rodri, as said before, was more like an important piece of a well-oiled machine. The star was the team. There was no drop in team performance when he was subbed off at half-time in the final, which is not a knock on him but rather a testament of the collective strenght of Spain.
Rodri is also facing a structural disadvantage: he can't match the stats or the entertainment value produced by attacking players. This has historically, and rightfully, been a very important factor in voter's minds.
Class and fair-play:
Rodri.
A huge minus for Vini if it's razor-close. A criterion of little importance though I guess. It won't help you win, but can disqualify you if you cross the line (cf. Zidane in 2000).
Someone has mentionned the Ronaldo-Modric situation in 2018. There are some similarities but also major differences. Modric had won the UCL that season, which is more valued than a league title. He also had a more impactful individual international tournament (WC, more prestigious than Euro) than Rodri. I don't think they could have given the Golden Ball to anyone else for example.
Ronaldo, unlike Vinicius, was very poor in the UCL final and SF. He didn't win the league. He was also a less exciting and complete player than Vinicius is right now. He was incapable of having a game like Vinicius had against Bayern in the second leg, despite not scoring. He couldn't do what Vini did to Kimmich. That version of him was long gone, he now needed to score to look good.
If it was Rodri vs Ronaldo 2018, Rodri would have my vote.
The Ballon d'Or 2024 could look like this:
1-Vinicius
2-Rodri
3-Bellingham
4-Carvajal
5-Lautaro
Then in no particular order: Mbappé, Yamal, Nico Williams, Foden, Kroos, Kane.