Yngve Johan Tirevoll provided beta on this route:
Vassfarstien was originally launched in 2017, by The Norwegian Trekking Association, as a proposed six day hike from Honefoss to Fonhuskoia (The Fonhus Cabin), via the aforementioned cabins. The reverse direction may be preferred for runners for logistical reasons, since it´s really hard to get back to civilization from The Fonhus Cabin. What makes this route hard, is its steep inclines and declines. While climbing is never required, keeping a fast pace is nearly impossible for long stretches of the route, especially between Veneli and Storekrak. Also, be aware that the original route went via Vassfarkoia (The Vassfar Cabin), instead of Veneli. However, Vassfarkoia was destroyed in a fire in 2018, which led to the official use of Veneli instead, cutting the distance from 116k to 113.1k.
The route is considered one of the cultural heritage routes of Norway, since the national park Vassfaret is of great interest due to its history, its appearances in national literature, art, and its wildlife, with regular sightings of bears and wolves.
A Norwegian resource website by the trekking association (DNT): https://www.dntringerike.no/vassfarstien