What you're looking for doesn't entirely make sense.
Vacuum ins't anything, per se. Vacuum is the absence of things, e.g. particles, pressure, etc. A perfect vacuum doesn't exist, thus generally one is interested in varying degrees of low-density fluids. Thus one would most likely rephrase that you're looking for a simulation of two fluids, with strongly varying density.
The standard method of simulating fluids is with either a grid based hydrodynamics code (a so-called 'Eulerian' code), or a smoothed particle hydrodynamics (SPH, a `Lagrangian' code). Grid codes have trouble simulating large differences in density (especially over small spatial scales), which suggests that you should try using an SPH code. In astrophysics, which is the field I am most familiar with, the standard SPH code is called Gadget (note that it can be used for simulations of small scale objects, instead of galaxies and larger---which it is generally advertised for).
If that is insufficient, you should google it, or be more specific.