там в мануале всё написано

Requirements
• To use external effects, you need audio hardware with multiple inputs and out-
puts. This is necessary because your external effect will require at least one
input and one output (or input/output pairs for stereo effects) - apart from the
input/output ports you use for recording and monitoring.
• As always, audio hardware with low-latency drivers is a good thing to have.
Nuendo will compensate for the input/output latency and ensure that the au-
dio processed through the external effect isn’t shifted in time. However, this
automatic delay compensation results in added latency - if your audio hard-
ware has a high latency to start with, this could be a problem.


Setting up
To set up an external effect device, proceed as follows:
1. Connect an unused output pair on your audio hardware to the input
pair on your hardware effect.
In this example, we assume that the hardware effect has stereo inputs and outputs.
2. Connect an unused input pair on your audio hardware to the output
pair on your hardware effect.
3. Open the VST Connections window from the Devices menu.

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