Wikipedia has a nice article, which I used as the basis for this simple model.
You can find an equilibrium at (Relative initial predators = 3, Relative initial prey = 0.5). An interesting experiment is to determine whether that point is stable.
Lotka-Volterra-Alt.vpm (an alternative version that enlarges the diagram slightly and moves most of the graphs to a control panel on a second view)
Another version of the model, which requires an advanced version of Vensim or the Model Reader, uses a subscript (array) to randomize the initial conditions for the predator and prey levels. That allows you to see the effect of the parameters over a variety of random realizations in the predator-prey phase space.
Lotka-Volterra+Array.vpm (requires Vensim DSS or Pro, or the free Model Reader)
Update: I wrote a bit more on this topic here.