Fusion of vegetative hyphae is known as

The fusion of vegetative hyphae is known as Anastomosis.
An anastomosis is a connection or opening between two things that are normally diverging or branching.
In mycology, anastomosis is the fusion between branches of the same or different hyphae. Hence the bifurcating fungal hyphae can form true reticulating networks.
Hyphal anastomoses represent key structures for the formation of efficient and interconnected AMF mycorrhizal networks, which are fundamental to AMF survival, plant/soil nutrient flow, and the maintenance of genetic diversity.