As for GTA Vice City, it was just nice to see some familiar faces still doing well for themselves. Interestingly enough, GTA San Andreas does a good job of adding some lore to GTA 3. All of these games were a part of the 3D universe, so it made sense when there was some crossover here and there.
Instead, some GTA San Andreas debuted in either GTA 3 or GTA Vice City and were later put in GTA San Andreas when it made sense for their role. Not all of GTA San Andreas' characters were created with that game in mind.