I had the same problem with my son when he was your twin's age. He would scream and turn blue if we travelled anywhere longer than 5 minutes. He was a big baby (at 3 months he was 8 kg, so we just turned ours around and into sitting position. He never cried in the car again.
If I can remember way back, the advice I was given was to wait half hour after a feed cos their tummies may become unsettled. But before sleep is good for longer trips, not shorter, otherwise you wouldn't want to have to wake them up or disturb their sleep if you have to stop and get out after a short trip. Then they may get really cranky and gang up on you, hehe.
Vaccines: Question what you're told...