I do know that the indoor stadium can seat up to 12,000 people, but that's mostly for sporting events. Besides, the set for the 20th anniversary tour was huge, I doubt it would seat 10,000 people. In fact, it was confirmed that they opened up 7,000 seats
(source). Star Vista's theatre is quite unlikely because, well, it's a theatre lol. I'm well aware that pop concerts have been held there before but I think it's unlikely that a dome show would be downscaled to the Singaporean equivalent of a hall. Especially when it's most likely gonna be such a large scale one. But I guess anything can happen.
Regardless, it's interesting to see that the organising team is still considering to add more dates/locations still. QUEEN OF CASHING IN THAT RETIREMENT FUND. I'll gladly pay more to watch Namie in Singapore, on top of the 2 Tokyo shows that I'm already going for.