Hi!
Not need to invert/cancel the signal.
From my experience, it appears that an external signal on channels 1/2 passes right through the Arturia VST to the output. I only use Reaper, so I don't know if it does that in other DAWs, too.
I found that setting the external signal to be on channels 3/4 (instead of 1/2), and then setting the VST to use channels 3/4 for input will make it work correctly. I haven't tried it with all of the Arturia VSTs (I'm not sure which ones can use an external signal), but I have gotten this to work successfully with the ARP 2600, Moog Mini, and Moog Modular. Once I moved the external signal from channels 1/2 to a different pair, it worked exactly as it is supposed to.
Actually, I found that using channels 5/6 also works. I don't feel like going through the effort to see if 7/8, 9/10, ... also work, but my guess is that any pair of channels other than 1/2 will probably work, as long as you set the external signal and the VST input to the same set of channels.
Note: I found a post with a similar question in the "ARP 2600" forum, and gave the same reply there :
https://forum.arturia.com/index.php?topic=95571.0