Here is the current comment about VMThread.

// A single VMThread (the primordial thread) spawns all other threads
// and is itself used by other threads to offload heavy vm operations
// like scavenge, garbage_collect etc.
class VMThread: public NamedThread {

it's not accurate. VMThread is different from primordial thread. I guess the comment is based on old codebase(git blame gives us 8153779ad32d1e8ddd37ced826c76c7aafc61894)

Because VMThread is singleton, I think it's also a good idea to hide Constructor and Destructor in "private" section.