Rocket propellants: accuracy of vapor pressure estimation techniques
I would like to determine the vapor pressures of a couple of substances (rocket propellants) over a significant temperature range within a ~10% error margin.


The best way to determine vapor pressures is to measure them.  Is that not an option for you?


  It is important that the approximation results in a closed representation of the vapor pressure curve , therefore I would like to avoid methods that have points of discontinuity (e.g. Antoine equation?).


The Antoine equation isn't discontinuous by nature, it's only discontinuous if you use two different parameter sets for a single substance in order to widen the temperature range of applicability of the equation.  


  What are your experiences with vapor pressure approximation? Could you provide some tips on how to improve my results?


I would recommend the Antoine equation as a first step beyond the Clausius Clapeyron equation.  Unlike the CC equation, the Antoine equation does not assume that the enthalpy of vaporization is independent of temperature.  It also provides a single, closed-form representation of the vapor pressure dependence on temperature.

Unfortunately, the Antoine equation requires compound-specific parameters.

However, many of the other methods you report using, such as the Lee-Kesler and Ambrose-Walton methods, also require compound specific parameters: specifically, the critical temperature and pressure of the compound of interest.  Even the Clausius-Clapeyron equation requires an (assumed constant) enthalpy of vaporization, which is also a compound-specific parameter. 

Thus, the Antoine equation is probably the best place to start.  As Wikipedia shows, it can have accuracy better than 10% deviation for some compounds.  Ideally you would measure vapor pressures for your two compounds and then fit the data to the Antoine equation.  If measuring the vapor pressure of your compound is not possible, you could start by examining the Antoine equation predictions for a series of compounds that approximate your unknowns in chemical structure and molecular weight.  However, with this latter approach, I doubt error under 10% will be possible.

Antoine equation parameters for your compounds of interest


UDMH has Antoine parameters available from NIST.
The case of dinitrogen tetroxide is complicated by the equilibrium $\ce{N2O4 &lt;=&gt; 2 NO2}$.  It is unusually difficult to fit vapor pressure data to.
Hydrazine has Antoine parameters available from NIST.
In general NIST has Antoine parameters for a variety of substances.