Talk to Pediatrician on New Born Sucking Problm

Plz tag lactation consultant and language speech therapist. My daughter born with 28 weeks. Spent 30 days in NICU. Now she is 1 month and 18 days. She caught pneumonia twice in 13 days (after discharge from NICU). She discharged on O.G and mother feed. She is unable to do enough sucking. Hardly sucks 1 minute consectively. She vomits every second or third feed in a day. Doctors advised me to keep her at 45 angle till 6 months. Her vomiting fears me about milk aspiration despite 45 angle. All medicines (gas, bloating, sugar enzymes, probiotics) done to stop vomiting but all in vain. What should I do to stop vomiting? When she will start sucking at her own? O.G might be source of germs transferring in stomach. Weight: 2 kg plus. Now she completed 35 weeks as per gestation age.