Are you suggesting that there could be a second video input (i.e. an external DVD player), and that the audio from it will be mixed with the microphone audio (which needs delaying to match the latency of the PTZ camera stream), and the mixed feed will come into vMix via a single (Tascam) USB interface? If so, that's no longer just "2 inputs - Video stream and Audio Microphone". Instead it's two classes of input, one of which needs an audio delay applying and the other which doesn't but, because you'd be pre-mixing them before taking them into vMix, you wouldn't be giving vMix a chance to apply different processing to each class.
The solution is to split the audio into two input groups (externally, e.g via the soundboard) - one for each class of audio - and then take them into vMix via two interfaces - or a single interface with more channels and an ASIO driver - and then you can add the delay to one audio input group in vMix and not to the other.