What is "enough Vitamin D" ?
Let's take the mainstream approach and look at IOM (Institute of Medicine) RDAs. They suggest 600 IU (15 mcg) for people under 71 and 800 IU (20 mcg) for people older than 71.
I have a software where I can see the nutrient content of foods and I have listed 130 foods that in my opinion are the most commonly eaten in western societies (probably not an accurate representation but that doesn't really matter much). When I order the list according to Vitamin D content, I get only 5 foods that reach or surpass the RDA (with 2400 kcal).
Two of those foods are mushrooms (15-20 mcg D would require 300-400 grams of Chanterelles or 5-11 kilograms of White mushrooms).
One of them is a fortified food, Kellog cereals, which would give you enough D if you ate nothing else at all (well, you could eat those 4 other foods).
The remaining two foods are fish (300-400 grams) and eggs (700-900 grams, or 13-17 eggs, for the RDA).
The list also has 16 foods that have at least some Vitamin D detected but those are small fractions of the RDA, mostly dairy and meats. Vast majority has 0 mcg.
It looks like the only food (other than fortified foods) with enough Vitamin D is fish (and maybe some mushrooms if you REALLY like them). I don't think many people eat a dozen eggs a day. You could realistically eat 300-400 grams of fish a day and I bet many people do (at least in some countries and professions).
I would argue that food is not a good source of Vitamin D, or even truly a source at all when you look at the average diet. You need the sunlight. Only takes a few minutes of UV-radiation from the Sun to get the RDA (in many places this requires summer and midday, winter and rainy season doesn't really work).
There's also lots of talk about people needing 50-100 mcg of Vitamin D a day to hit proper blood levels, not just 15-20 mcg, and from the studies I've read, I tend to agree.