We just updated the news page with more information of Forest Pack 3, you can see the new features working there.
Replying your questions:
1. Yes, Forest 3 will be compatible with VRay. It now can use Vray proxies as custom objects, and convert the trees to instances of the proxy object (see the demo video).
2. Yes, the price of the upgrade will be Eur 125, except for licenses purchased until six months before the release date of Forest 3, that will be updated without cost.