Wallace (if I understand him correctly) is not saying that ὁ διδάσκαλος καὶ ὁ κύριος are morphologically vocative (which they are clearly not), nor that they are nominatives used as vocatives, but merely that in a sentence of the type “you call me X” the second direct object X should be in the accusative. Whether the irregular use of the nominative instead of the accusative is “because of the special character of the individual described”, or is simply an example of bad Greek depends, I suppose, on your theological position.
I would suggest that the formulation in John 13,13 represents a mixture of two constructions: “you call me the teacher” with “the teacher” in the accusative, and “you call to me (saying) ‘Teacher!’”, where the word for “teacher” would be either vocative or nominative.
I could add that the ancient translations treat these words unambiguously as vocatives. The Vulgata has : “vos vocatis me Magister, et Domine (voc.)”, and the Pshitta has : ܐܰܢ݈ܬ݁ܽܘܢ ܩܳܪܶܝܢ ܐܢ݈ܬ݁ܽܘܢ ܠܺܝ ܪܰܒ݁ܰܢ ܘܡܳܪܰܢ (literally: “You call to-me our-lord and our-master”.)