First of all, the number of subscribers does not tell the whole story. Check out the traffic stats for more information. In March there were 3.7M unique visitors to this subreddit.
Unfortunately those stats do not include demographics, because your argument doesn't really care how many people are here, but rather how much money they are investing.
Without looking at any real numbers, I would assume that this sub has a correlation to reddit's site-wide traffic. That is to say we are mostly young males. A lot of us have good income, but most of us don't have enough years to have invested much. Additionally we see a ton of posts from people who managing debt and are not yet at the stage of investing.
Again, just making assumptions, but take those 3.7M visitors and consider that maybe half have ever invested anything. And maybe a quarter of those 1.8M have actually taken action based on anything they've read here. And out of those 400k, maybe half are choosing Vanguard over Fidelity and Schwab (which are also promoted here, but not as much). And out of those 200k who invest with Vanguard because of this sub, maybe half would have done so anyways by reading other finance websites. So let's take our extremely theoretical 100k Vanguard investors that r/pf has generated and assume that most are early on in their investing careers, with an average of $10k invested. That's about $1 Billion of investments, which is less than 1% of the massive increase your chart shows over the past year.
"The reddit effect" is nothing to ignore when you're talking about traffic, but I think it's a pretty big leap to say that it's actually made a significant effect on index funds. The bottom line is that the majority of people here probably read and don't take action.