Before Vienna, the Allies had beaten Napoleon, invaded France and occupied Paris. Nevertheless, they decided to negotiate a settlement instead of dictating it.
In 1919, Germany was a fully developed democracy. If the ruling coalition had sat at the table in Versailles, the big popular parties (SPD, Zentrum) would have been the ones who negotiated it - and thus would have felt bound to observe it.
That the terms would be harsh, everybody in Germany had anticipated. Being treated like criminals, nobody had anticipated.
Guess what,the Entente public doesn't give a flying f$ck about Germany being a fully developed democracy.They've spilled a lot of blood and they want something back.You really can't negotiate anything if you have no bargaining chips.In 1815,the various powers have already dictated the peace by deposing Napoleon.Arguably,forcing the French off most of their gains and taking their emperor out is already a form of dictation.There was no negotiation for that.
Agree completely with darthfanta, although slap on the wrist for (probably unintentionally) aggressive tone!
Its worth noting that the SPD regime is hardly what might be called legitimate in this period - coming out of the chaotic collapse of Wilhelm II's government. Even if they were legitimate 100%, the Allies are under no obligation to invite them. The Congress of Vienna analogy is a little phony in my opinion - technically the Bourbons, not France, were present as the negotiators and it was only the sly powerhouse that was Talleyrand that prevented the terms in 1815 being harsher.
Also, why would they look back to 1815 as inspiration? For the French the obvious inspiration was 1870 and the humiliation of the Franco-Prussian War. You've got to remember, on top of revanchism, that France loses so much in the war - between 4 and 5% of the pre-war population are dead, much of Eastern France is chewed up and ruined, and out of the pre 1914 population about 1 in 10 were wounded in some way. They called the French generations of the 1920s and 30s "the hollow classes" for a reason! For many of the French Versailles doesn't go far enough. Quite a few wanted to properly dismember Germany (which, remember, had only been unified less than 50 years ago).
SO to get a milder Versailles I think you need to change the position of France. My best guess would be if France had been knocked out, or somehow more badly bruised, by the War. Maybe the army mutinies see the Western Front collapse? If the Allies still win in this scenario (which admittedly becomes a big IF) then France will have much less say at whatever equivalent peace treaty happens.