It is well-established that a wire reflector --on or just above the ground-- directly under a dipole can do just that.
I don't have a reference, but at a field day that I was present at years ago, a ham on 75m did that. Afterwards, as W8JI (who also was present) said, "He was just 'killing people'", meaning that his QSO count immediately went way up. His reflector was directly under his center-fed dipole. IIRC, it was laying directly on the ground. Although the spacing between the wire and the dipole was not optimum, it certainly seemed to work.
It was good and conductive soil, in NW Ohio near the Maumee River where Route 235 ends at a SW-NE road parallel with it. 200 years ago, that was part of the Great Black Swamp. Later, ditches were dug and tiles laid to drain it into the river, which turned all of Lucas and Wood county into fertile farmland. Still, adding that wire helped his signal.
Perhaps the band conditions improved at the same time, and the wire had little or nothing to do with it. But I've heard of too many similar experiences to discount that.
As for a vertical as you describe, I'm leaving that for someone else to answer.
See this answer by R. Fry
, which seems to disprove my anecdotal statements.