My daughter, now 18 months, also had only 2 vessels in the umbilical cord and we were quite worried as well. I had extra level-2 ultrsounds and then weekly non-stress tests in the last month. The ultraounds revealed no other defects, and she was born perfectly healthy, although smaller in birth weight than was anticipated. It seemed like her rate of growth slowed in the last month, which may or may not have been related to the missing vessel. In the 8th month, they predicted she's weigh 8 lbs. or more, but she was born weighing 6 lbs., 9 0z.
From what I recall my doctor telling me, normally there is 1 vessel (a vein) delivering blood and nutrients to the baby, and 2 arteries that take waste away. When one is missing, it is one of the 2 arteries, so the issue can be removal of waste, rather than diminished delivery of blood. I just tried to eat healthy, organic food so there wouldn't be as much bad stuff that would need to be eliminated.