Extracting Vertices / Snapping to points from Vector Tile Service (VTS) Layer

You can read the design principles of native vector tile support from https://github.com/qgis/QGIS-Enhancement-Proposals/issues/162

Vector tiles are placed on a special vector tile layer and geometries do not behave like true vectors. I guess that's why you can't snap to vector tile features.

A big problem with representation of vector tiles as vector layers is that they are not "proper" vector layers. Vector tiles are data that have been modified for efficient visualization, making compromises elsewhere:

Geometries have been split, clipped, simplified, features may have been filtered out altogether at some zoom levels. Having a polygon in one vector tile, we do not know if that is the complete representation of the feature or if the feature continues in some adjacent (or distant!) tiles. We cannot find a feature by its ID - we would need to browse through all tiles. We do not know how many features there are or cannot show attribute table of the whole sub-layer without fetching all tiles. All of these limitations of vector tiles come from the idea to make their rendering as fast as possible. We should not try to use them for other than display purposes. It is better to think of the data encoded in vector tiles as raw data for map renderer, not "true" vector data for analysis or other common GIS uses.