Several news outlets and popular science publications have reported on this subject in the last few years.
Often, the focus is not specifically on vitamins, but on nutrients in general (i.e. "phytonutrients").
One study often cited is Donald R. Davis' publication from 2004 that showed that 6 out of 13 nutrients had declined (statistically significant) in 43 fruit and vegetable crops during the second half of the 20th century. It is of course not the only study that has been published on the subject, and SFGate (2006) reports on a number of them, including reviews.
A common explanation is that we have been selecting varieties focusing on growing large crops in a small amount of time, while taking out some taste that most find undesirable. For example, we worked hard at getting rid of bitterness, which often is a sign of the presence of beneficial anti-oxidant chemicals (NewScientist published an article about it in 2015, sadly paywalled).
Other reasons can be the micro-nutrient depletion in agricultural soils, the over-supply of macro-nutrients like nitrogen, general soil health – including presence of beneficial micro-organisms, and smaller root systems.
As far as vitamins go, Scientific American mentions declines in vitamins B2, C, A. The New-York Times tells the story of breeding beta-carotene (precursor of vitamin A) out of corn.
Another article by Donald R. Davis was published in 2009 in HortScience and explains three observations that have been brought up in research looking into nutrient depletion:
- Inverse relations between plant yield and mineral concentration
- Apparent nutrient declines in historical food composition data
- Side-by-side comparison of high- and low-yield cultivars
He concludes that "the broad evidence [for minerals, vitamins and protein decline] is consistent with more definitive studies and seems difficult to dismiss".
Plant food grown organically are often seen to be richer in nutrients than their conventional industrial agriculture counterparts, most probably because of many things: less intensive, heirloom varieties, more diverse nutrients, healthier soil, more developed root system, etc.