The iphone setting is confusing. Being able to select and enable 5G standalone does not mean that you are using it.
Ok, then how do you know it on an iPhone that it is indeed connected to 5G SA?
From my iPhone, both M1 and Vivifi are connected on the n78 band, given that M1 is SA only, this can mean that Vivifi being on the same band should be the same, although earlier, ST is also using NSA on n78.
My earlier experience with Vivifi before I ported back in again, SA option on iPhone is greyed out, meaning no SA option that can be turned on. After the recent port in, SA option is available.