You are correct that the grapevine starts somewhere, but if it were just the MSM there would be no conservatives. And that number was all media.
Now, without going into my full diatribe, let me explain some things. The vast majority of any population will actively and deliberately ignore the news. Left, right, center. Doesn't matter, they don't give a fuck beyond maybe voting 'their team' if an election occurs. These people lack any hard ideological lens though and are nominally what you'd call the Center. Center-right, Center-Left, independent, doesn't matter they all fall in the Center.
Occasionally, things make them Take Interest though. These have to fit some fairly stringent criteria and utterly lack a partisan aspect. Failing to do so is why Hunter Biden's laptop went nowhere. When something makes them Take Interest, they will suddenly shift that attitude of deliberate ignorance and begin voraciously eating up any information they can on the issue at hand.
This is where the grapevine kicks in. The origin point can be.... basically anything really. A blog post could do it, hypothetically. And once it spreads it -spreads-.
What the media does so well is not actively shaping a narrative, but passively discouraging the grapevine by providing info to politically aware which can pass into the grapevine. But this is very ineffectual and is a large part of why modern MSM is dying.
Addendum: It's also worth noting that a massive chunk of all political tactics, on both sides really, is focused around simply avoiding triggering that Take Interest point. Once it occurs there is jack and shit anyone can do to prevent the info from spreading, taking hold, and the populace from growing agitated over it. The focus is entirely on lulling people to sleep and avoiding the trigger ever occurring in the first place.
Hence why, despite almost a year of concerted effort, views of the 2020 election as fraudulent have only increased over time.