Short and simplified version is that VAC checks that the parts of the RAM that is being used by the game isn’t being modified by a malicious program such as cheat engine.
Luckily VAC isn’t so good at this in that it only bans you if the program that is altering the memory matches a known signature in some database.
So as long as the midi playback program merely reads the midi file, encodes it into text and then sends it directly to the OS as a keypress then all should be well.
Unless the guy that made it decides to add a 360 noscope aimbot function and steam finds out about it, in which case nothing will be well.
As for playing in the songbirds store.
Playing with macros and midi playback software in your own condos is ok in macs eyes:
It has also been stated before that since the condo server that you are hosting is being hosted by your own computer, you pretty much get the final say on rules and what is allowed in said condo server.
However when you play on the servers hosted by Pixeltail you gotta follow their rules:
Which states that automation software such as keypressers, bots and macros aren’t allowed, which means you aren’t allowed to use automation software to play on the piano in songbirds.
Although if you’re lucky and ask them nicely they might decide to make an exception to allow it in songbirds, although i wouldn’t bet on it.