The updated, +:, and :+ methods ignore the provided CanBuildFrom instance, likely for performance reasons. This breaks type safety:

scala> (Vector(1,2,3) :+ 4)(collection.breakOut):List[Int]
java.lang.ClassCastException: scala.collection.immutable.Vector cannot be cast to scala.collection.immutable.List

The implementation of List.+: gets it right though, with minimal overhead (one instanceof check). I've attached a diff that applies the same implementation to Vector. All tests pass when applied to current trunk (277dc7cf43566f8294bde4143107d9bfaa59e8e3)

Obviously, performance may be affected, although an instanceof check should be cheap compared to the required node allocation, etc. The original code seems to have been added on 2009-10-14 with commit 1747692434cece862d63a0f67decd810707b1508 and includes the comment: TODO: check overhead of builder factories for methods updated, \+: and :\+. The comment was removed when the code was moved from IndexedSeq to Vector (commit 6f7723bea494da2616edc1877d2402d356787512). I wonder if any overhead checking was ever done.