Vaginal prolapse is and end result of straining which could be caused by several factors including kidney /or and bladder infection, low grade peritonitis,infection in the uterus,large size calf at full. Treatment can follow many routes but must be done by Qualified veterinarian. Success is possible only if the prolapsed vagina isn't soiled and not dry by the time vet arrives who will follow many procedures including suturing. Kindly seek help from vet nearby e.g from government offices too.