I think its both.
The letter made him not take the path we see Silco take, as a result the Underground isn't as dangerous so VI isn't as mature so early so, so she isn't as cautious and that results in someone getting themselves killed.
Monokuma
2024-11-27 01:26:08 +0000 UTC
There are 2 ways of "how we got a good universe"
1. What the guy on chat said, Silco found a letter, and job going slightly different, and Vi getting killed in the explosion is some kind of butterfly effect
Or
2. Vi death IS the fork. Back before their parents died on the bridge Silco was more or less the same "uncle figure" as Vander was for the girls...at least he should have been considering them 2 and girls mother were a friend-trio...Vi sudden death, espetialy so early on could have been a cold shower moment that made him change his ways last second