Input := rv0 = NestList[ f, vtest, 300];
Input := ListPlot[rv0, PlotJoined -> True, AspectRatio ->1]

Output = -Graphics-
Think about what the spiral outward from {1,0} means. The solution behavior is oscillatory but the energy in the solution is steadily increasing. Here is another way of looking at it. We produce a plot of the xn as n goes from 0 to 300.
Input := ListPlot[Transpose[rv0][[1]], PlotJoined -> True]

Output = -Graphics-