First
The VITamin D and OmegA-3 TriaL (VITAL) is a research study in 20,000 U.S. men and women investigating whether taking daily dietary supplements of vitamin D (about 2000 IU) or fish oil (about 1 gram of omega-3 fatty acids) reduces the risk of developing cancer, heart disease, and stroke in people who do not have a prior history of these illnesses. Recruitment for the study will begin in January 2010.

I really don't understand why this is an either or situation.

It may be just because it's a trial and they want a simple answer but correcting both omega 3 and vitamin D status are anti inflammatory agents and both will work together to improve the situation.

While 2000iu will probably (if they use D3) be sufficient (particularly if participants also take the extra 800iu/d they are allowed) to make a difference in the USA, in the UK where we don't have fortified milk/cereals but we have more cloud and fewer days UVB annually, UK adults would need more to achieve the same 25(OH)D and probable benefits.

With regard to the section
In October, the government stopped a big study of vitamin E and selenium pills for prostate cancer prevention after seeing no evidence of benefit and hints of harm.
again UK readers should be aware we are in a totally different scenario. USA people live for the most part in with selenium rich soils and we don't.
Biofortification of UK food crops with selenium
Se is an essential element for animals. In man low dietary Se intakes are associated with health disorders including oxidative stress-related conditions, reduced fertility and immune functions and an increased risk of cancers. Although the reference nutrient intakes for adult females and males in the UK are 60 and 75 mg Se/d respectively, dietary Se intakes in the UK have declined from >60 mg Se/d in the 1970s to 35 mg Se/d in the 1990s, with a concomitant decline in human Se status. This decline in Se intake and status has been attributed primarily to the replacement of milling wheat having high levels of grain Se and grown on high-Se soils in North America with UK-sourced wheat having low levels of grain Se and grown on low-Se soils. Animmediate solution to low dietary Se intake and status is to enrich UK-grown food crops using Se fertilisers. Such a strategy has been adopted with success in Finland. It may also be possible to enrich food crops in the longer term by selecting or breeding crop varieties with enhanced Se-accumulation characteristics. The present paper will review the potential for biofortification of UK food crops with Se.

The research showed that low selenium levels were associated with increased Prostate cancer incidence and thus our low selenium levels still need correcting but in the USA giving extra selenium to people who already are getting more than enough aggravates certain gene variant prostate cancers and makes them worse. So both too little and too much selenium will increase PC but in the UK were we are all Selenium insuffiicient it's a different story.