Thoughts on MapBox's Vector Tiles
They're nice. The servers are fast and the content is good. Given that these aren't explicitly designed for mobile use, they work pretty well anyway.
If it were me, I'd add another couple of levels in dense areas. Newer hardware can power through the level 14 tiles, but it tends to strain older devices.
Can I Use These?
No. Mapbox hasn't released the tiles and there's no way to pay for them. You shouldn't make an app with them. Don't be that guy. No one likes that guy.
Nonetheless, they work great in Maply and if you were a big enough customer I'm sure you could work something out with Mapbox.
Why did I bother? Believe it or not, this isn't the first set of Mapnik vector tiles I've supported.
Maply Vector Tile Support
Other people are doing their own Mapnik vector tiles, standing up their own servers, and generally doing their own thing. Some of them are my customers and some of them have even shipped apps.
The Maply support for vector maps is in good shape, but there are a few things I'd like to improve. Labels could be better, particularly the font twiddling and layout modes and there are some bugs worth tracking down.
Mapnik vector tile paging is in WhirlyGlobe-Maply 2.3.