Doffy is probably strong enough to get mid diffed by the weaker admirals while being decently above the current strong vice-admirals. I say current because there is no such a thing as vice-admiral level. They can go from Maynard to Garp(in the past). With Tsuru who was even portrayed to be above Doflamingo's level in the past. So there is nothing humiliating for Doflamingo to be a vice-admiral. He'd probably be the strongest vice-admiral in the current marine organisation, but not strong enough to be an admiral