I'm afraid your child is too young for a Ride Safer Travel Vest. The older one can use it but a baby at a year is MUCH safer rear-facing. The older one really shouldn't be in a booster so look into it for him, since he's only at the minimum age. This is recommended by car seat techs over a booster for a child his age.

http://safetrafficsystem.com/ver4/

Are both parents traveling? If you're organized, bringing the car seat shouldn't be a big deal. Do NOT counter-check it. You might want to invest in something lighter and easier to travel with, if your car seat is especially large and cumbersome.

With two adults and only two children (evenly matched!) you can manage it, especially since there are no safe alternatives. Perhaps you'll get lucky and can use the car seat on board for the baby.