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To understand the problem, it suffices to read the source code of the plot_vector_field function:

sage: plot_vector_field??

As you can see, the first line of the code is

(f,g) = f_g

where f_g is the first parameter of the function, which means that it assumes that f_g is either a couple of functions (as in (lambda x,y:x+y, lambda x,y:cos(x))), or a couple of symbolic expressions (as in (x+y,cos(x)), not a a single Python function with 2-dimensional output.

This explains your problem, and i consider this as a bug, or at least a missing feature.

Thanks for reporting, it is now trac ticket 22115.