Though you are correct that the filling of the tank will displace vapors to the exterior of the tank, a lit cigar or cigarette is not a competent ignition source for ignitable liquid vapors. Lighting the cigar with a match or lighter is competent however.
In addition, testing of any residual liquids after a combustion reaction will generaly not give you an accurate reading of the flash point of the liquid before the incident as the lighter ends of such liquids will likely have been burned off or evaporated. Such testing, as well as GC, may be able to show that the "Diesel Fuel" was contaminated with a lower flash point liquid.
Pat Kennedy, CFEI, CFPS, MIFireE
Fire and Explosion Analyst
Sarasota, Florida
[
www.kennedy-fire.com]