What I'm doing is doing the Portland missions until I unlock Staunton Island. Don't do Asuka's first mission yet as the mafia will hate you after the mission. After unlocking Staunton, it allows you to get vehicles for the Import/Export garage and the harbour crane in Portland, since some don't spawn in Portland. Do taxi missions, done. Hose down 20 fires on Portland, then Staunton, then Vigilante missions 20 times each on Portland and Staunton, then do Paramedic missions on Portland. Also the TOYZ missions and then the rampages, and the checkpoint challenges. But always do the El Burro missions and Marty Chonks' phone missions. There, now you're basically almost done the side missions of the game. Just kill 20 criminals and put out 20 fires on Shoreside Vale and you're done. Then start doing the Shoreside Vale import/export garage, then any left side-missions.
I know where some vehicles are for the Portland import/export garage.
1. Blista, in parking lot by Asuka's condo.
2. Mule, in parking lot near import/export garage.
3. Bus, driven in usually Aspatria, Staunton Island.
4. Coach, At a bus depot in Portland down the road near Joey's garage.
5. Securicar, I haven't found one myself, but is supposedly driven in North Staunton near the stadium.
6. Mr. Whoopee, during the firefighter side-mission, sometimes you put out fires of the ice cream truck. End the mission and steal the truck.
7. Dodo, this is one where you need to go to the airport and drive carefully through Portland since the only way to unlock the airport is do the storyline, which upsets the mafia and triads.
8. Trashmaster, usually driven near Kenji's casino and the Love Media building.
9. Patriot, in the parking lot at Supa Save!, to get this one, I just start the checkpoint challenge, then do one checkpoint to start the timer, then take it to the garage.