Simple alternative to UNIFAC to prove that a mixture containing 1ml of x and 5 litres of propylene glycol is not flammable?
UNIFAC is not a procedure, unless by chance in a similar field two things got the same name, which is unlikely.

UNIFAC is used by activity coefficient models for predicting liquid properties. It is often coupled with a vapor phase equation of state, and quite often simply the ideal gas law...

A flash point calculation is a procedure, and must be done in whole. Note: you can do either an experimental flash calculation, or a computational one!. UNIFAC is used in the latter. It combines the fact that at equilibrium a specie in both liquid and vapor must have the same chemical potential (gotten through via activity coefficients in the liquid phase and fugacity coefficients in the vapor typically) coupled with mass balance. You cannot do half of it and have an answer... like all calculations it must be done fully and correctly. It is not that difficult, but it should be done by someone who is a scientist or engineer... This is the type of thing that would be expected of a chemical engineer.

Most chemical engineering Thermodynamics textbooks have a decent section covering flash and bubble/dew point calculations.

There are many programs that can do flash calculations for you. VMGSim, Aspen/HYSIS to name just two of the many commercial/industrial simulators plus a plethora of small programs online made by people like me (mine isn't online ;) ).

I just did a quick google and this looks like it would work https://checalc.com/solved/multi_flash.html

Note, you do not need to use UNIQUAC in the liquid, but, it usually by default will use UNIFAC to fill in unknown interaction parameters, so it is basically a safe activity coefficient model. Depending on what is in the vapor, the peng-robinson or even ideal gas equations of state are probably good enough. If there is association or the molecules are polar, then I recommend the Haydon O Connell.

The link I sent looks like it uses the Peng-Robinson equation of state for liquid and vapor. It is probably okay for what you are doing. Actually the link I sent doesn't look like it has PG in its database... this one may... https://www.daoftx.com/usingflsc.htm

But you know what to look for now... These premade programs just need you to tell them the species, initial composition, T, P and then you just hit calculate. You just need to find one that has your molecules in its database.