At a certain point in the story Brucie will send you 1 email at a time with details of a certain vehicle's that he needs. If you reply to the email you can go to the location that's specified on your map to get the vehicle and after you take it, you have to bring it to the garage in the harbor near Brucie's house. You have to get 10 vehicle's for the achievement.
When you reply accepting the mission you will need to do it at that moment.
You have a limit of 40 inbox e-mails, so try to erase some of the first mails you get so more can appear in your inbox.