Vue really is a powerful foliage program, but maya can create forests just as efficiently now with the paintFX section of maya, it's a little hard to understand at first, and I'm not sure about maya8 but maya7 doesn't support mental ray with paintFX, unless you convert to polygons but then your scene gets HUGE, so you'll have to composite your forest into your scene in post production. (assuming you're main lighting system is mental ray).
Check out the
paint FX tutorials on AREA (it's a free registration to access tutorials)