You're fine, just dont have IM open while playing any game with vac, preferably any game at all.

Every now and then IM for me stop working in that way. I fix it by making sure my phone does not have steam open, checking again, if still not working then I restart steam and after that try restarting the computer. It usually gets fixed after just closing and opening IM with checking my phone inbetween.

Try on another computer if you can just to check if its the installation(s) (steam/im)

The "very rare" I think is really just a way for the devs to not guarentee that you wont get VAC banned. One way VAC works, from my understanding, is that when you launch a game or join a secured server, VAC scans the Vram allocated to running programs. If a certain program's signature matches that of a detected program or application, then VAC takes action. Becuase VAC is not client side (installed on your computer), your computer has to scan the programs, upload the signatures to Valve, check them against banned/untrusted signatures, and take action from there. That's probably why there was a 10min delay. There is also a lot of Machine Learning at play with VAC I belive, but that's the jist of it!

TL;DR: It means you cant use it while playing a vac secured game, you're fine.

This is a known cause of Idle Master. Valve added "Idle programs" to their untrusted list of applications a while back. Two good rules of thumb is: 1) Don't play VAC secured games while Idling 2) Don't play games while idling

EDIT: Oh, yea! Also read the pinned discussions!
Edit again: https://support.steampowered.com/kb_article.php?ref=2117-ilzv-2837&l=english#software Read the last bullet point.