I think that hole is too small for a vole. I've seen voles, they're as big as the smallest mice.
That said, it's really tough to judge distance in a picture like that without something for reference.
What's the diameter of the hole?
Could it be a sweat bee hole?
If you've got sweat bees in your garden that should be a boon!!!
I don't know if that's what you got, but that's what's I found in my little hole.
This was in the lawn last year.
My husband put up a little posted pink flag near the hole so I wouldn't step on it. ha ha
I'd say it was less than a half inch wide... the hole itself I mean.
Not all sweat bees are green like this one, and they may vary in types in different regions. (I'm in PA.)
Append: Squash bees also dig holes like this (I remember I wanted them even more last year because my zucchini was a big disappointment because of poor pollination.)
Also, you could have a causation problem... Your idea that the holes have something to do with the poor crops nearby... Could be that the same thing causing those crops not to do as well, could be the same thing that made the spot attractive to whatever's digging the holes. Just a possibility.