What is the difference between the neural tube and the dorsal ventral cord?
The terminology can be confusing, and I suspect the question you meant to ask was: what is the difference between the neural tube and dorsal nerve cord?

In vertebrates, the neural tube and the dorsal nerve cord are two names for the same structure. It develops into the brain and spinal cord.


  Neurulation occurs during the early embryogenesis of chordates, and
  it results in the formation of the neural tube, a dorsal hollow nerve
  cord that constitutes the rudiment of the entire adult central nervous
  system.


Source: Colas J-F and Schoenwolf GC. Towards a cellular and molecular understanding of neurulation. Developmental Dynamics. 2001. vol 221(2): 117–145.

This differs from invertebrates, some of which have a ventral nerve cord that does not invaginate and form a neural tube. As the name suggests, in contrast to vertebrates the nerve cord in these species is located ventrally.

In summary:
Dorsal nerve cord = neural tube
Ventral nerve cord ≠ neural tube