I don't think there's anything you could spray on the poison ivy that wouldn't harm the hedges.
What I would do is get all dressed up in haz-mat gear - long sleeves tucked into gloves, long pants tucked in to boots - and then cut the poison ivy vines maybe a foot up from the ground, being sure to leave some leaves on them. Then use a brush to PAINT RoundUp on the cut surfaces.
Painting it instead of spraying it, minimizes the environmental harm and keeps it from killing the hedges. The reason for leaving some of the poison ivy, not cutting it off at the ground, is that it needs to uptake the RoundUp and circulate it into the roots.
You can leave the parts of the vines above the cuts until they are thoroughly dead. They still need to be handled with care, not touched, but they will be a little easier to manage.
Just a couple reminders: Bag the poison ivy up in plastic and send it to landfill. I hate to do that, but there's no other safe way to get rid of it. Do not put it in compost; the oils can spread through all the compost. NEVER burn it. The oils in the smoke can lead to whole body exposure which is horrible or if breathed in, can lead to internal poison ivy, which can be life threatening.