Amazing, thanks for explaining.You need to go to the Import/Export Warehouse where those vehicles are stored when you return from doing the glitch the car will be there. Dont look in the menu at first. Then you do the steps to drive out the warehouse, then destroy it (i drove into water) then call insurance get it replaced, then go back to warehouese, force a save. just for curiosity and i dont think you need to do this bit, i then went back to story mode and then back to online and they were still in the garage and also showed up in the SV menu.
Although I'm finding it difficult to find my vehicle warehouse, never had to use it before haha