The only real way to convert them is to use a program like 3ds max. Ped skins will have to be rigged to the SA skeleton and the only good way I know to convert cars without bugs is with 3ds max/anos script and TXD workshop. I did a tut for a guy on converting cars from VC to SA. Check it out HERE
. As for ped rigging, there are hundreds of tuts for that on youtube and on this forum in the TUTORIAL section.
EDIT: Nevermind, I got the from this game to that game backwards! But you will still have to rig the SA skin to the VC skeleton. As for the cars, there are other topics/tuts on that here. Just search for them.