Encouraged by @PamNor's experience (while @rinigus originally sounded quite discouraging), I tried vector maps on a Jolla 1 phone (with SFOS 2.1.4.14):

mapboxgl-qml 1.3.0-10.35.1.jolla is working properly with both, WhoGo Maps 0.92 and Laufhelden 1.0.0-1, no matter if using online or offline vector tiles served by OSM Scout Server 1.4.3-10.29.1.jolla!
With OSM Scout Server as map provider, using vector maps seems to be a bit faster than than OSM Scout based bitmap tiles (in Poor Maps; I have never tried Mapnik offline bitmap tiles due to their enormous map sizes).

Kudos to you all, this is quite an achievement.