That map doesn't show any "principle trade routes" going through Vilnius. I'm not saying you're wrong (there was obviously trade going somewhere) just saying it's not evidence for your suggestion.
Short answer: Balance, probably. Gameplay > History.
Long answer: I wouldn't think of the nodes as "all trade in node X goes through that province first, then out of it" but instead all trade in node X flows towards node X's outlets. Of course this conflicts somewhat with interpretations of trade power and COTs, but it's a reasonable interpretation of trade nodes.
In your map, the route going through Kovno(?), near Vilnius goes to Novgorad and Prussia. So the current situation is about half right: trade in the Kiev node goes to Novgorod, which would be appropriate for Vilnius, but the other downstream node of Poland is not appropriate for Vilnius. This situation seems ok because obviously the node graph can't capture every provincial situation perfectly.
On the other hand, your point about the river also makes good sense, so an alternative could be adding the Baltic as an outlet for Kiev. I don't really think that's a good idea. Gameplay-wise it'd drain even more out of the node. History-wise, looking at the rivers and routes on your map, there's a clear divide between rivers/routes in the Baltic region and rivers/routes toward Kiev. This conforms with the current node configuration of Kiev going to Krakow and Novgorod. Vilinius is obviously on the Baltic side of that divide so it would be reasonable to place it there.
Either way, I think you can make a reasonable case for Vilnius in either the Baltic or Kiev nodes. In the end though, gameplay wins.
And if you're serious about the suggestions, post this in the suggestions forum.
