I had a 6 month spell with both rubbers within the last 1-1.5 years, so I think I can add my 2 cents or a recommendation for an 10% H2SO4 or HCl enema.
I would strongly agree with laistrogian expressed.
Vega Japan felt a touch firmer. less control on lower to medium impacts. Much more difficult to make opening topspin slow and loaded heavy vs a dead or light underspin ball. Easier to loop drive. Top end had noticeably more pace and a little more spin. Throw was lower than Pro. Could counterloop an incoming fast with power. Crappy (lack of) feel on most impacts, except for very well struck impacts, shined there. To use Vega Japan, you gotta be ballin' on every shot or you will crash.
Vega Pro was softer sponge, much easier on lower or medium impacts, had much better feel. Very easy to make spinny, slow heavy topspin vs a long serve return. Less pace on loopdrives. Top end less penetrating, but still spinny. Higher throw. easier to control incoming loopdrives with changes in grip pressure. Soft topspin defense much easier. Could make some shots out of position if you go through the ball a little. Overall less power, but way more control across a wider broadband of shot types. Keeps the ball on the table with much easier ability to use spin and speed variation.