Worked well for me at a network buffer of 512 samples (on a 100mbit network using CAT 10 crossover cable), so realistically you're looking at 768 samples total latency (inc soundcard). I occasionally got it to work at 256 samples but it was the luck of the draw. I heard reports that gigabit networking can achieve lower latencies (maybe because packet headers travel more quickly?). In any case, I could get about 10 stereo tracks across at this latency.
You don't need Vstack - the server is a host in itself. You can however host a subhost such as EnergyXT or Chainer within the server, allowing you to perform some of the mixing there, saving bandwidth.