If the problem is the VRay efficiency, a good trick is to disable the filter options in the opacity map. Usually, this improves the rendering speed, without quality loss.
Btw, have you take a look at the HQ Plants in our website ? The full package includes tons of high quality trees and bushes, ready to be used in VRay, at a very affordable price. Furthermore, the next week we are going to offer a coupon discount for all Forest Pro users. We'll announce the campaign in the forum when ready.