My eldest had only 2 vessel cords. She was fine, no problems whats so ever. The doctors just kept a close eye on her. They said she might come early but she came right on her due date. She was and still is healthy birth conception
The joy of the medical technology right now is, your doctor has caught this early and is watching it. Your baby will be fine, if small at birth, and will catch up on the nutrition he didn't get later.
If his heartrate is still good, and he's still growing a little, he's fine and will eat a lot and catch up after birth.