Pickens would not power all vehicles w/ NG nor does he claim to zero out foreign oil imports. Pickens proposes the following:
Currently 22% of US electric power comes from NG. He would take all of that NG producing electricity, replace it with wind generation, and use it instead as transportation fuel. If successful that would reduce foreign imports of oil by 38% and thereby allow ~$300B/yr of the total $700B/yr going to foreign oil suppliers to stay in the US.
Given the recent opening of shale NG fields, the US could keep this up for at least a of couple decades given its domestic NG reserves, which are now twice the size of US oil reserves (per unit of energy).