What "existing popular VMs" are you talking about? Was there a widely spread VM before JVM? Perl and Python are older than Java, Ruby should be around the same age.
which one? Squeak first appeared around the same time as Java. was there a free, open-source, solid Smalltalk VM back when Perl, Python or Ruby were started?
The original Smalltalk VM was not open-source (the concept was not invented back then) but was very well documented and had some different implementations.