Mri indicated enlarged vessels in brain and likely adult onset hydrocephalus but more recent csf flow study indicated no detection of adult onset hydrocephalus--why the discrepancy?

When the lateral ventricles appear enlarged in an older person, it may be due to central atrophy, or sometimes Normal pressure hydrocephalus (NPH). Classic symptoms of NPH, are gait disturbance (trouble walking) often confused with parkinson's, Dementia, and Bladder control issues. The CSF flow study suggests it's not NPH, but no test is perfect. If symptomatic I'd suggest further evaluation.

Gather that you intended ventricles instead of vessels. As we age, the brain does slowly shrink and the ventricles expand as a consequence. This is variable and does not always represent pathologies such as Alzheimer's or normal pressure hydrocephalus. Talk to your physicians about their interpretations of the imaging studies.