If the vomit was yellow then it was bile. This often occurs in the morning when the dog hasn't eaten in a while. It is natural for this to be expelled. Try feeding the dog a handfull of food and a little water before bed, or wake up a little earlier and feed.
Hope this helps

Aaron McDonald
Animal Behaviorist;K-9 Security Consultant
McDonald K-9 Academy
Birmingham, AL
(205)595-6642