If there’s blood in my baby’s vomit, should I take her to the emergency room?

Yes! One possibility is mallory-weiss tear: http://www.Nlm.Nih.Gov/medlineplus/ency/article/000269.Htm.
Call your doc first. Depending on the age of your baby and what is going on, this may or may not be a medical emergency. In the first few days after birth, it is not uncommon for babies to swallow maternal blood from fissures in the breast. If your breasts are cracked, it could be due to that. Your doctor can help you with that. If there is fever, large amounts of blood in the vomit, severe pain, the er might be best.