Malls have historically been background in GTA games. Trailer 2's mall section goes further than that - a full interior, multiple floors, packed with NPCs.
The shopping districts piece covers the wider retail vocabulary.
The mall scene in Trailer 2
A multi-level indoor area featuring railings, glass-fronted stores, escalators. A food court can be seen. A crowd of NPCs of different ages and fashion styles. Rendered as a real interior environment, not a flat 2D background.
The implication: the mall itself is a building you can enter. It's uncertain whether it's open from the beginning of the game or unlocked later in the story. It's also uncertain whether all of the stores inside are interactive (typically in GTA, most are just facades, with only a few being real).
Where the mall sits geographically
Trailer 2 doesn't offer a definitive exterior view of the mall itself. Community readings have placed it in different parts of Vice City; we don't commit to a location. The mall is plausibly suburban - massive malls have historically sat on the outskirts of cities - but its visual coding is much more akin to a flagship downtown mall you'd find in real-life Miami.
Multiple malls are plausible. Real-life Miami has Aventura Mall, Dolphin Mall, Dadeland Mall and Bayside Marketplace. A city the size of Vice City isn't going to have only one mall.
What the mall is likely to be used for
The trailer's mall scene includes what appears to be a robbery beat. That's consistent with how GTA has used malls before: high-density retail means hostage and heist content. We're guessing on the details, but the framework is based on previous games.