After further inspection, I noticed the two characters at the bottom bare a close resemblance to Colonel Cortez and his right hand man Gonzales. In GTA Vice City, Cortez was forced to leave the city after running into problems with the French, and his no.2 Gonzales was killed during the mission "Treacherous Swine". This means that - if these two characters are definitely who I think they are - then Vice City Stories is set prior to GTA Vice City (late 70s, early 80s perhaps?). Also the fact that Gonzales has a full head of hair in the boxart (whereas in GTA Vice City, he doesn't) supports this theory.
Cortez & Gonzales
In addition, saintdave from our forums noticed a similarity between the girl that's standing next to the car, and Laura Shrub:
Mrs. Shrub is the wife of congressman Alex Shrub (who featured in the mission "Martha's Mug Shot" in GTA Vice City) and as you can see in the comparison picture above, the hair style and body build etc. is exactly the same, so Mrs. Shrub will - without a doubt - make an appearance in Vice City Stories.