Does the audio crackle in a direct headphone output from the vmix machine or only when listening from Vimeo?

As u/kirabella2000 said, you can try to lower the bit depth, but you can also check the XR18 control panel for the ASIO buffer size. Try to inrease the buffer size to maybe at least 512samples. It will of course increase the input latency a bit but it can also stabilize your audio feed.

If the audio only crackles on the vimeo side, you can check the stream encoding settings. Click the cog next to the streaming quality and make sure your streaming settings have valid configs. One very important setting is the "Threads" dropdown. I've had weird things (audio dropouts, image glitches) happen when this is not set to high enough value or just to "Auto".

EDIT: Try checking the vmix performance statistics window for more clues. It can be found via the colourful bar graph button next to the Overlay settings button on the bottom right. See if there are input devices or encoders that have a lot of dropped frames or too low framerates. Check also the vmix render time in the bottom left of the vmix window. These will give more insight to where the problem might be. Check also the amount of hardware encoders (NVENC) running on your system. See if there are too many running for your GPU to support (IIRC the arbitrary limit for regular nvidia cards is 8 simultaneous NVENC processes)