The possible deal, while representing only a beginning phase of European integration for Ukraine, is a watershed opportunity for the country to free itself once and for all from the former Soviet yoke.
The deal should have been signed much earlier in President Viktor Yanukovych's tenure, but the high-profile prosecutions of former Prime Minister Yulia Tymoshenko and her ally Yuriy Lutsenko provided a justification for already-reluctant members of the EU, led primarily by Germany, to back away.
EU diplomats are now billing the Vilnius summit as a "last-chance," and there are good reasons to believe that if the signing is derailed, the deal will be off the table for a number of years, if not forever.
This would negate an outstanding body of technical preparation work by Ukrainian ambassador to Brussels Konstantin Yeliseyev and his team.
Ukraine's foremost supporters inside the EU, Poland and Lithuania, cannot hold out indefinitely against the heavyweight nations of Western Europe, who are strongly disinclined against any suggestion of further EU enlargement.
Among the European elite, friends of the Ukrainian cause such as former Polish President Alexander Kwasniewski and former head of the European Commission Romano Prodi have been advocating tirelessly on Ukraine's behalf, but can hardly be expected to pull any more rabbits out of their hats.
No one understands the impasse in Ukraine-EU relations better than Russian president Vladimir Putin, who threw down the gauntlet earlier this month at a meeting with Yanukovych outside Moscow.
For historical reasons, the Russian leader has long been eager to stop Ukraine's drift westward and pull it back into the linguistic, cultural and psychological construct of the so-called "Russkiy Mir" (Russian Universe).
Putin called Ukraine's possible entry into a new supra-national organization with Russia, Belarus and Kazakhstan a question that is "political in essence," tacitly acknowledging that any economic benefits of a new Russian-Ukrainian union are only of secondary concern for him.
This stance directly contradicts Yanukovych's long-held position that cooperation among former Soviet nations should be primarily economic. The president and other Ukrainian government officials have frequently noted that the most important economic advantages of post-Soviet re-integration could be achieved by a simple free trade agreement with Russia, making a union with political characteristics superfluous. However, despite a relatively weak hand vis-a-vis Yanukovych, Putin holds at least one ace: he knows that there is more popular support in Ukraine for a new union with Russia than many observers in the West would prefer to admit.
Although nationwide polls have indicated a slight preference among Ukrainians for European integration, the numbers favor the customs union among retired voters and in heavily-populated regions of southeastern Ukraine - the same demographics from which Yanukovych will need overwhelming support to win reelection in 2015.
For this reason, it is dangerously naive for the West to assume that Ukraine is already "in the bag", and that the free trade and association signing can be withheld as a punishment for Tymoshenko's imprisonment. As such logic goes, Yanukovych's occasional hints that Ukraine could join the customs union if it is rejected by the EU are only a bluffing tactic.
However, this view ignores the likelihood that, in the event of full customs union accession, Putin would offer Yanukovych a sweetheart arrangement that protects the latter's personal and family interests.
If his drive to move closer to Europe fails, depriving him of a major success on which to base a national reelection campaign, Yanukovych will have to show some sort of alternative achievement to his voters in Russophone Ukraine in order to remain in power.
And that is why Putin and his top political ally in Ukraine, Viktor Medvedchuk, are reportedly expecting to Yanukovych to be on board with the new union no later than the beginning of 2015. Such an outcome would be tragic for current and future generations of Ukrainians, as it would lead to the Belarusization, if not a wholesale breakup, of the country.
Regardless of which language they speak, Ukrainians are unanimous in their desire for better governance and higher living standards. The historical records of the past 20, 30, and 50 years all show clearly that it is the European Union, not a new Soviet Union by another name, which has the best chance of delivering these improvements to Ukraine over the long term.
This is less a matter of geopolitics than of basic self-interest and economic common sense. Unfortunately, with so much at stake, the debate over the signing of the free trade and association agreement has become too much about the Yanukovych regime, instead of being about a long-term European strategy toward Ukraine as a whole.
The effects of the upcoming decision by the EU will reverberate for decades after the current president's departure. Putin's most powerful argument in favor of the Eurasian Union is not positive, but negative: he asserts that Ukraine is unwelcome in Europe, and therefore Yanukovych will have no choice but to turn eastward sooner or later.
"Sooner" almost happened on Dec. 18 of last year, when Yanukovych cancelled a high-profile visit to Moscow just minutes before the scheduled departure, at which he would likely have promised to join the Customs Union in exchange for cheaper Russian natural gas prices.
Putin is apparently so confident of Yanukovych's eventual capitulation that he was unwilling improve the terms of his proposal to get an immediate signature.
As former President Leonid Kuchma once famously stated, Ukraine is not Russia. But perhaps more importantly in today's context, Ukraine is also not Belarus or Kazakhstan - a pair of incorrigibly authoritarian regimes where civil society and the media have even less freedom than in Russia.
Taking this latter fact for granted would be a mistake that Europe - not to mention tens of millions of Ukrainians - might end up regretting dearly down the road. The EU should sign the free trade and association agreement in November and use ratification, a process which is likely to be drawn out over a number of years, as the conditionality stick.
Better safe than sorry.
Will Ritter is a freelance writer based in Kyiv.