$> myjava --add-modules=jdk.incubator.vector -XX:+UnlockExperimentalVMOptions -XX:+EnableVectorSupport -XX:+PrintFlagsFinal -version | grep "EnableVectorReboxing"
     bool EnableVectorReboxing = false {C2 experimental} {default}
WARNING: Using incubator modules: jdk.incubator.vector
openjdk version "21-internal" 2023-09-19
OpenJDK Runtime Environment (fastdebug build 21-internal-git-d67bfe21a23)
OpenJDK 64-Bit Server VM (fastdebug build 21-internal-git-d67bfe21a23, mixed mode, sharing)

PrintFlagsFinal prints false for EnableVectorReboxing, which actually is true during runtime. It is changed at https://github.com/openjdk/jdk/blob/49ff52087be8b95cbf369518281312ecc9d83618/src/hotspot/share/classfile/modules.cpp#L463 .