I'm so fortunate that I never have to leave my boys alone. When I'm working (off on sick leave now), I work eves and my hubby works days. So, they always have one of us here. If we're going to be gone all day and can't take them, my daughter watches them for us.
I have had to be in the hospital a few times, and it was very hard on Jake especially. My daughter took care of him, but she said he seemed sad a lot. But, as long as they are with someone who will spend time with them and take good care of them, they are ok...they just miss us when we're not there. I left a shirt with my scent on it, and she said Jake slept with it when he napped and at night. You might try something like that.
Have a great vacation!