Vdom as explicit proxy

Dears Could u please advise about configuring vdom as explicit proxy. We have fortigate 100D and we created one vdom includes two interfaces one as wan and the other as wan. Then we have configured it as explicit proxy. The point is we are keep getting the error 504 dns lookup failed fortigate. Could u please advise if this vdom can work by this way as an explicit proxy. And how to set the dns for it. I can see DNS setting under global tap only. Best regards