Is vomiting a prominent symptom of nec?

Yes, but. While vomiting is seen with nec, there are so many other causes of vomiting in babies, that it is difficult to use this alone as an indicator. More ominous signs of nec include blood stools, abdominal distension, lethargy, and, occasionally, temperature instability.
Yes. Nec or necrotizing enterocolitis is an infection of the intestine in infants especially premature infants. It can cause damage and even perforation which would result in stasis, increasing gastric residuals, and bloody stools.