This might occasionally work but more times than not, I see people doing this and the vehicles remain empty in the base. Sometimes the opposing team comes up with a rush while a player is buying vehicles for others this way.
Realize that upon production, the vehicle is locked for 20 seconds for the purchaser, but not for the opposing team. Many times this caused a drawback; the opposing team rushes, few of their vehicles get killed but the drivers rush towards the emty vehicles and continue the rush.
To prevent such happenings, I make it a habit of putting timed C4's on the empty vehicles after i entered it. If it takes more than 30 seconds for someone to enter it and join the advance, it's too late. But imo a vehicle destroyed is better than handed to the enemy.
I think the difference is that I'm organizing a rush. Like calling ppl to enter those vehicles and go rushing with a group. Leaving empty vehicles is dumb.