Assuming you have found a repack named something like GTASA_Vita_Working_Repack.vpk (and potentially a separate data folder), follow these steps:
Inside ux0:/data/gtasa/ (or similar), find sa_defs.txt. Open it with a text editor on your PC. Adjust these values:
Look for the San Andreas bubble on your live screen. Tap it. The first boot may take 30–60 seconds (building shader cache). Do not press anything. If you see the Rockstar logo and hear the iconic theme, you have a working repack.