Prime still is missing a lot of feature which were common in Mathcad 15 and below. Plotting vector fields is one of them. Vector fields in MC15 is, while being two-dimensional, part of the 3D-plot component. Plotting in general is really mediocre in Prime and 3D plots are even worse and not worthwhile.
You may try a routine I once wrote for doing vector plots, direction fields, etc. using just the normal 2D plot (the MC15 3D vector fields had some drawbacks, too). I also converted that worksheet to Prime 2 (the current version at that time) and you should be able to use it in P3.1, too.
The worksheet can be found here: Vector Field Plots in 2D for Mathcad 15 and Prime 2
As you wrote you tried to convert a MC15 sheet, it seems you can read and convert MC15 sheets - here is another usage of those routines -> Direction Field of Lotka-Volterra ODE | PTC Community
Regards
Werner