If Russia was really confident, they would have turned the spigot off on Ukraine already. They have not done so yet. My guess is that the Russians really need every gas sale they make just to keep going.
But, the Russians also have an advantage. Putin is not only well, relentless in his pursuit of Russian interests, but is also highly intelligent. My bet is that though Russia would get hurt more than the EU in an embargo, none of their leaders and nobody in the Obama adminstration has the nerve and experience to call Putin's bluff and then out play him.