Am I right that the files under test/fixtures/a/ are NOT in the Mapnik Vector Tile format ?
Mapnik Vector Tile files should start with hex 1A 93 (tag version=15, value 2), and indeed they do in the Mapnik Vector Tile testsuite.

But these files under tilelive-vector start with hex 78 9C, and the code even recognizes that 2-bytes sequence to mean "pbf". What's that ? Is it the OSM file format ? Is tilelive-vector supposed to be able to parse MVT encoded vector tiles at all ?

See also mapbox/mapnik-vector-tile#48 and mapbox/vector-tile-spec#20 for additional confusion ...