Ok, thanks for the advice.
So I guess somethign like
initial_conditions = reduce(vcat,[collect(mass.s .=> [0.0, 1.0, 1.0]), collect(mass.v .=> [1.0, 1.0, 0.0])])
is the way to go isn’t it ? When multiple vectors have first to be collected, then gathered together through reduce
EDIT : This was actually brought up in 2D arrays with ModelingToolkit - #7 by baggepinnen