I am not the founder of the glitch. DRObythePounD420 has some credit if this ends up anywhere other than Reddit.
Auto-Sign in needs to be enabled on your console.
Go online(public or private), and pick a good spot.
Change armor and wait for the little save icon on the bottom right hand corner of the screen.
Go into sp, get your vehicle
Find a good spot where the vehicle will likely not despawn after loading a save.(A good spot is the parking lot right next to michael's vinewood garage.)
Make a quicksave (If you need to check if the place you are using is good spot, then load that quicksave right here and see if it despawns.)
After you have made your quicksave, go online(public or private)
click ps/xbox store, you should be on the "are you sure you want to go to the ps/xbox store" screen.
As SOON as you click X/A to confirm to go to the ps/xbox store, press your center button, Account management, hit triangle, and sign out as soon as possible.(It is ok if you see the ps store slightly open before it tells you that you have been disconnected)Press the center button to exit the xmb.
Spawn in sp, Load that quicksave
Once your save has loaded, press center button and sign in.(xbox users sign in and load save)
Once you have signed in, Press down on the dpad and go to your online character. It should be on the screen that says "are you sure you want to quit gtav?"
As SOON as you click x to confirm to go online, press your center button, Account Management, hit triangle, sign out, then press the center button. The xmb should close and immediately open the sign in screen. Click X quickly to sign in, then it should bring up the joining gta online screen.
You should see the vehicle where you parked it with your Sp character
Good luck, I hope that helps! Works 100% for me!!
I figured out how to land in a "private" lobby with your friends, instead of jumping into a crazy random lobby! http://www.reddit.com/r/gtaglitches/comments/264jex/bring_cars_into_private_lobby/