Not sure which version of Vue you have (ecosystems? you talk about hand placing so I wasn't sure), but I think you'll have more success using grass objects rather than solidgrowth plants.

Here's what I was able to come up with, I made an ecosystem material using Walli's grass pack 2 - the three grass patches, setting density to 100%, maximum size variation to x=0 (y & z are grayed out), drove the density with a crisp black and white image created in photoshop. The edges aren't perfectly square because the patches are somewhat rounded and irregular.