Yeah it's pretty straightforward. Do all the standard standard social media stuff and hype it up to your friends, but that's one percent of your effective advertising or less. For Vimeo and many other things your main choices are online advertising via Google Adwords (for hardcore people that want the full HUD), facebook, and youtube ads.
It's not as expensive as you might think and it's very effective, however ROI is a challenge and the rates you pay are highly dependent on a number of factors such as content type and quality.
Finding the right match for your project would typically involve months of A/B/C testing across multiple platforms, but would result in dramatically increased viewership. After three to six months of actively testing campaigns, You'd be more experienced and better equipped at promoting any type of online venture. I would say that ultimately it's worth it, and something that filmmakers should do more rather than less.
I've told people this 1000 times and I'm fairly certain that no one has ever listened to me, But its standard practice in the industry to spend close to half of your entire budget for a film on its advertising. Sometimes studios will only do a 3rd or a quarter depending on the film, but one of the great failings of the indie film industry is that a huge majority of indie filmmakers set aside somewhere between zero and 1% of their total budget for marketing.
If a tree falls in the forest and no one hears it, did it actually make a sound? Yes it made a really loud sound that no one heard and had absolutely zero effect. My advice is to take a crash course in online marketing via informational youtube videos, then invest in advertising cautiously as you gradually learn the ropes and how to make your spend effective.