Is walked a vivid adjective?

No. It is neither vivid nor an adjective; it is the simple past tense form of a descriptive verb.

To be an adjective, it has to modify a noun; to be vivid, it has to create a striking picture in the reader's imagination. 'Walked' does neither.

An example of a vivid adjective would be 'rushing', as in rushing ants: it modifies the (plural) noun 'ants' and gives the reader a mental picture of an ant colony darting about at random.