Andy,
There is info on the VA website listed as "Benefits for Veteran's Children with Birth Defects." Basically, I believe all you have to prove is your father's service in Vietnam, your birth after his service and your diagnosis of Spina Bifida. You should be able to prevail with your claim. Keep us posted on your claim as it progresses. Hopefully, your efforts may eventually help other children of AO vets.
Good luck and take care.
GP