OK for starters youre going to need txdworkshop or imgtool (these programs can be found in thegtaplaces downloads section)
with txdworkshop (or imgtool) open up gta3.img you will find it in your gta vice city models folder and find textures you want to replace take the txd file and press export as bitmap. When you do this just open the texture you exported and open it with paint and then reskin it however you want it.
Then save the texture and open up gta3.img and find the texture you exported now press import and import the texture you reskined then press save,close the program and play vice city and just find your retextured cars,weapons,buildings,people.
And one more thing i told you how to retexture but you can ask more people how to model with zmodeler and 3dsmax.