People sometimes forget Vita is basically on par with the iPhone 5, same SOC, just 1 extra core (not used in games anyway) and much lower clocks.
Imagine GTAV running on an iPhone 5. The most powerful sandbox it ran was probably Gangstar 2 or 3.
I never played the Gangstar games, but they were up there in terms of mobile tech. (Gameloft had a history of really pushing phones with "console-quality" graphic techniques... and then, they also made games with those graphics too, I guess?) They put money into their phone games, and when you played them, you realized where they ended up skimping (even without the MTX, Gameloft gameplay always screamed phone-quality to me, plus all the auto-triggering and rubberbanding to make cool shit happen more often in order to make up for unwieldy touch-for-buttons controls,) but some of the games were actually good and no matter what, the impressive engines made them
look like they would be good games.
It's too bad that Gameloft ran out of interest quickly with Vita and 3DS. (They did an Asphalt for both plus a Dungeon Hunter on Vita, but that was about it I think?) I think ports of their games could have been welcome on these platforms, and GameLoft had a lot of the console'ish franchises (clones, really) in genres that ended up being underserved on these two platform.
This is Gangstar Vegas from 2013, so from around that time, and it ran on an iPhone 4 even. I never played it, but it looks to me somewhere in between the PS2-era GTAs and the PS3-era GTAs, with modern effects and character modeling but I'm guessing a lot less environment detail and polygon budget. (It basically looks like a Vice City engine on steroids at first glance.)
So I don't know if this is any indication whether GTAV could have scaled all the way down to a Vita much less 3DS (there's an experiment where somebody
cranked GTA5 to super-low settings.) But eh, none of that happened and so I'm fucking sad about my Vita and 3DS all over again! Long live the Switch, at least...
