There's a few reasons. The main difference is that Vimeo uses a higher data rate than YouTube: ~2.6Mbs vs ~2.1Mbs -- about a 20% difference.
Then there's the source. First, Most Vimeo content is produced by creative professionals so the content itself is meant to look amazing. In contrast, YouTube does bulk video, most of which is homemade. Vimeo looks better on content alone.
Second, the Vimeo video is directly encoded from the creator. YouTube's creative content tends to be commercial work posted via distributors and agencies. It's likely that the content shown on YouTube has been encoded twice - first by the creator for the distributor, then again by YouTube's uploader.
There's other things. It's probable that Vimeo is using 2-pass encoding, allowing for more accurate VBR. It takes longer, but looks much nicer. YouTube users want speed, so one-pass may be standard.
As an aside, have you personally witnessed better quality using 2-pass encoding? At ~2mbps, I've never really noticed anything in terms of quality... but in the 6-8 neighborhood, I can def tell a difference.
Of course! If it's just a "talking head" then two-pass is useless. But for anything that changes between action and static content (like movies), two-pass is necessary to get useful VBR: extra data overhead from the low-action can be passed on to the action sequences while maintaining the same average data rate.
Generally, the first pass is to determine which sections are static vs. which are busy. The second pass then takes advantage and can adjust the rate. So for long talking head scenes, it may drop the data rate to 1.2 Mbps, and action scenes can be ramped up to 3Mbps+ so the action doesn't look blocky.
Then there's the source. First, Most Vimeo content is produced by creative professionals so the content itself is meant to look amazing. In contrast, YouTube does bulk video, most of which is homemade. Vimeo looks better on content alone.
Second, the Vimeo video is directly encoded from the creator. YouTube's creative content tends to be commercial work posted via distributors and agencies. It's likely that the content shown on YouTube has been encoded twice - first by the creator for the distributor, then again by YouTube's uploader.
There's other things. It's probable that Vimeo is using 2-pass encoding, allowing for more accurate VBR. It takes longer, but looks much nicer. YouTube users want speed, so one-pass may be standard.