Sick doggie

[pittsburghirish]

Hello,
I have a two year old border collie mix named Bandit. When I came home tonight from work he didnt greet me in his usual rambunctious way. I let him out side and he didnt want to come back in. This is even more unusual. So I was thinking that he was down because he was home alone alot today so i took him on a walk. While walking he vomitted a white liquid that looked like slime. Anyways, ive been sitting with him and he seems very tired and there is a slight whistle when he exhales. Im going to take him to the vet tomorrow but please somebody tell me how serious this sounds. Id like to get some sleep tonight but now its not looking like it. Any knowledge would be greatly appreciated.

Thanks.

Larry