I never bothered with diapering my girls during their heat cycles. The natural way of cleaning is the best for them to keep clean. If you hinder that process in any way they never learn to do it the right way.
The only time I use any type of cover up is when we were preventing a pregnancy.
The throw up could be stomach bile. (is it yellowish and frothy) It so here is a site that may help if this is the case.
Why is My Dog Vomiting Yellow Liquid?: Causes for this Chronic Form of Canine Stomach Upset | Suite101.com