While in general, mixing Scala versions can cause NoClassDefFoundError, I’m surprised about this case, because scala.Function1 has been at the same location for many versions, so I don’t see how one can get that error. Or did name mangling change?
It should be noted that if you drop a JAR in the lib folder, SBT does not resolve dependencies for that JAR. On the other hand, if the needed dependency is on the classpath, it should be fine, at least if the code uses normal classloading. Is perhaps some custom classloading taking place?
I would experiment: maybe put the standard library in lib. Or try to publish vireo to a local artifactory and load it from there. Instead of “sbt run”, try to package your app and run it outside of SBT. Check the code the custom classloaders.