I wanted to update the sp2mp glitch working with lag switch. To guarantee your vehicle being the one you want you need to use the save editor. Now for each specific car that you want, in the save editor select the character you will be using and change the current vehicle to the one you want to bring online. When you switch to that character they will spawn with the car you upgraded in the save editor, not needing to quickly try to run for it.
For PS3 this still requires a lagswitch, and at this point still a second person. I will update this as I have a few ideas I am trying for a solo method.
Mods I started this one because the other sp2mp thread is spammed with debates.