Deer might slow down an approaching coyote.
A few weeks ago, I began calling, and two deer at about 400 yds got nervous, and crossed a large ravine, making a hasty exit. I saw a coyote headed my way, and he happened to come through the spot where the deer had vacated. Coyote spotted the moving deer, and watched them intently, until they disappeared over the next ridge, several hundred yards from him. He was there several minutes, and no amount of calling, and call changes would make him look my way. When he could no longer see the deer, he continued in my direction. The coyote was very leisurely and deliberate the entire time. When the coyote dropped into a low area, I took the opportunity to check out my immediate surroundings, and discovered a coyote standing looking my way at about 100 yds. I took this coyote, and never saw the other again. I'm sure he booked at the shot.
I have had deer leave, and I have had them approach my calling. Rarely do they ignore the call, but that has happened too. From this one incident, I would say that deer between you and an approaching coyote, could considerably distract the coyote, and it might take him longer to get there. When he spotted those deer in flight, it was just like your four year old glued to a cartoon! He couldn't take his eyes off them.
Yes, the deer will spot a coyote before you do, like deerthug says. Pay attention to EVERYTHING!
By the way, while deer hunting, I have seen deer chasing coyotes away, when they got too close to a group of them.