I am planning to drive from Vilnius to Riga between Christmas and New Years Eve. I would like to know how the road conditions usually are in this time of the year? Especially on the smaller roads. And is there anything special to consider when driving this route? Here is the map. (Recommendations for stops/sights would also be very welcome)
Given that you're going at a time of year where the high temperature is below freezing, you can expect some ice. However your route in Lithuania follows primarily "main roads" and should be well maintained if the Lithuania road administration is to be trusted.
Where I'd be cautious is once you're into Latvia, where the fatality rate for driving collisions is amongst the highest in Europe. Most of the roads you're traveling on should however be plowed. The one exception might be your scenic tour along the coast, which you might consider skipping in exchange for a straight shot into Riga along the E77 if the chance of snow is high.