  In this paper, we establish a partial order principle, which is useful to
deriving vector Ekeland variational principle (denoted by EVP). By using the
partial order principle and extending Gerstewitz's functions, we obtain a
vector EVP for $\epsilon$-efficient solutions in the sense of N\'{e}meth, which
essentially improves the earlier results by removing a usual assumption for
boundedness of range of the objective function. From this, we also deduce
several special vector EVPs, which improve and generalize the related known
results.
