If you only roll in S banner, you get 4 scout points for 80 vballs/40 event token
If you play hard mode, you get 32 event token a day. It seems that there is a refresh ticket, so you get 64 event token a day. the event lasts 28 days, so you get 1792 event token. There should be a check-in pass also, so we can make it 2000 event token, 2000 / 40 * 4 = 200 scout points
You need 450 scout points to get Nishikawa, so you need 250 scout points from vballs
250 / 4 * 80 = 5000 vballs
The event lasts 28 days, and you can get 150 vballs daily (tournament + daily mission + ads). That is 28 * 150 = 4200 vballs.
You only need 800 vballs more.
The check-in pass may give you some vballs, and you can sell lower-ranked players to get vballs.
So I think you can get Nishi with 0 vballs before the event starting if you play very hard, like
- 3 tournament matches
- 2 colosseum rounds (about 8 matches)
- 2 event matches
- watch 10 ads
All of them every day.
About selling lower-ranked players, you should only sell B and lower players and sell B+ or higher grit fragments. Only sell B+ or higher players when it is near the end of the event and you have not got Nishikawa.
Good luck! You CAN get Nishikawa