There seem to be a number of questions circulating on this but none that really answer what I'm looking for so here goes:

I am making a river in my map, and the blend texture that I want to use (sand & grass blend) on the displacements has no default grass growing out of it. So my question is this: How do you create or edit a VMT file so I can make it have grass on it? Does anyone have anything that's not too complicated, because that would be great. Thanks.

I suppose you're talking about the detail grass sprites. You know, The little grass that pops up when you get closer, sways in the winds, etc.

To add grass to your texture, you need to define the detail type that's being used.

Add this to your VMT for that texture:
"%detailtype" "flash_grass01"

"flash_grass01" is a reference to an entry in the detail.vbsp file, which defines the different grass sprites to be drawn. There are several other options to choose from that will get you different blends of grass and different grass densities. I dont quite remember where that detail.vbsp file is, but do some poking around and you'll find it (might be in the dods gcf).

to follow up, here is an example.

This is a VMT for a blend of grass and gravel that I'm using in my map:

"WorldVertexTransition"
{
"$basetexture" "ground/grass001"
"$basetexture2" "nature/gravelfloor004a"
"%detailtype" "flash_grass01"
"%tooltexture" "nature/blendgrassgravel001a_tooltexture"
"$surfaceprop" "dirt"
"%keywords" "dod"
}

Notice the line that defines the detailtype

Cool, thank you very much.