Snapd is downloading without my permission

snapd does not have the ability to disable its autorefresh function. Merely the option to choose at what times of the day it can do it.

This is more of a workaround than a proper solution, but if VLC is the only snap you have, you can simply remove the snapd package and install VLC via the standard apt install method.

Though a matter of opinion, I prefer apt installing my software anyway, as it is more space efficient with disks. The downside would be possible dependency issues in some less common cases. Snaps eliminate this problem by having everything required for a particular program in one snap.