Academic researchers often go looking for concurrency bugs,
and when investigating the Java libraries, the Vector(Collection)
constructor is usually the first one they find. E.g.

http://www.cs.sunysb.edu/~liqiang/atomicity-introduction.htm
http://www.soe.ucsc.edu/~cormac/papers/tldi05.ps

Pretty much any user expects that, since Vector is synchronized
and "thread-safe", then the following is safe:

Collection c = new Vector();
// Mutate c wildly
Vector v = new Vector(c);

Of course, Vector cannot assume that its argument is not being
concurrently mutated. Fortunately, c.toArray() is perfect for
initializing Vector's own internal array.

This is part of the fix for
6347106: (coll) methods taking concurrently modified collection arguments fail sporadically