I don't know the AsileFX "method", but it's relatively easy to change the foliage in Vue. There must be a ton of tutorials on the matter too, in case you feel the need. I don't read/watch many tutorials myself, but one good one I remember, is about making two-coloured leaves (top & bottom) with material layers, at Geekatplay, don't have the exact adress/link, but it should be easy to find.
I suggest you study the original material of the species you want to change, most of the trees require a "twig" texture, but that can be made in Photoshop or similar image editor.
The simpler plants can be given new leaves etc. with a few mouseclicks, once you have texture maps with a corresponding alphamap/channel - as the XFrog bitmaps.
It's great fun and very satifying to make your own stuff, I think, so have fun!