Does emitting the signal cause the QVector elements be copied?
No. The implicit sharing documentation is also clear about it:
Implicit sharing takes place behind the scenes; the programmer does not need to worry about it. Even in multithreaded applications, implicit sharing takes place, as explained in Threads and Implicitly Shared Classes.
Just to highlight the first paragraph from that part of the documentation:
Qt uses an optimization called implicit sharing for many of its value class, notably QImage and QString. Beginning with Qt 4, implicit shared classes can safely be copied across threads, like any other value classes. They are fully reentrant. The implicit sharing is really implicit.