  We first study Z(N) walls in a deconfined phase of Witten's D4-brane
background of pure SU(N) Yang-Mills theory, motivated by a recent work in the
case of N=4 SYM. Similarly to it, we propose that for a large wall charge k ~
N, it is described by k D2-branes blown up into a NS5-brane wrapping S^3 inside
S^4 via Myers effect, and we calculate the tension by suitable U-duality. We
find a precise Casimir scaling for the tension formula. We then study the fate
of Z(N)-vacua in a presence of fundamental flavors in quenched approximation
via gauge/gravity correspondence. In the case of D3/D7 system where one can
vary the mass m_q of flavors, we show that there is a phase transition at T_c ~
m_q, below which the Z(N)-vacua survive while they are lifted above the
critical temperature. We analytically calculate the energy lift of k'th vacua
in the massless case, both in the D3/D7 system and in the Sakai-Sugimoto model.
