The best work-around I have found is to mount up my main in whatever "vehicle" and simply have the slaves follow that. I seems that follow stops working for a toon that IS mounted in a "vehicle" but seems to stay intact if the follower is not.

Once I got my cold weather flying, another tactic that has worked has been to mount up the main in whatever the quest vehicle is (like sitting on the shoulder of a titan) and having my two slaves mount their fliers and have one follow the other. Then, I just fly the two slaves manually (well, one manually, one on follow) to the area where I need to be, and then bring in the main. The fliers stay out of danger, yet get credit for the kills, leaving me to single-box the encounter with my main. Luckily, most of these "take over something and kill a bunch of stuff you couldn't while alone" vehicle-type quests seem to allow this "flying within range without participating" completion.

It's annoying, but it works. Hope that helps.