Yes, when I first saw the ban I also thought that this was the most likely cause. But like I said, none of the accounts that my account is shared with have VAC bans. As far as I understand, if they got me VAC banned then they'd also be VAC banned.
Here's a screenshot of the accounts my account is shared with
And here are all those accounts
https://steamcommunity.com/id/naturebow/
https://steamcommunity.com/profiles/76561198018454015/
https://steamcommunity.com/profiles/76561198061332685/
https://steamcommunity.com/profiles/76561198089587697/
Ah, I didn't mean to say that I would be banned because Stardew Valley was VAC protected. Rather that I was playing it through the modding API (that I think uses DLL injection) while Dota 2 was running. Thus causing it to get picked up as some sort of cheat for Dota 2.
I've heard conflicting reports. A lot of people say the same thing you're saying here, that you have to be using Cheat Engine on the VAC protected game to be banned because of it. But there are people who say they have been banned just because they had Cheat Engine open in the background and have then been unbanned by a support agent saying not to run Cheat Engine while playing a VAC protected game.
So I'm no sure which one is actually the case. However, as I haven't used Cheat Engine on Dota. It would have had to have been the background case for me.
Thanks to everyone whose replied thus far. I'll probably try bumping my support ticket again, but I fear they'll just ignore me or close my ticket. Though I guess that wouldn't really be any different from me not doing anything.