New study below .
It was over 10 years ago when I reviewed the vasectomy-PCa literature. Some studies showed a link; others didn't. I remember three things: (a) distaste for the organizations that tried to spin the data, so that men would continue to have vasectomies - the health of the men was clearly not a priority; (b) evidence that risk increased over time & perhaps did not appear for at least 20 years, & (c) that young age at vasectomy might be a factor (which is probably the same as (b)).
My vasectomy was well over 35 years in the past when I was diagnosed with PCa, but, over time, more studies were published that found no link. The endless meta-analyses naturally found no link too. End of story.
& then in 2014:
"Vasectomy and Risk of Aggressive Prostate Cancer: A 24-Year Follow-Up Study" 
"Among 49,405 US men in the Health Professionals Follow-Up Study, age 40 to 75 years at baseline in 1986, 6,023 patients with prostate cancer were diagnosed during the follow-up to 2010, including 811 lethal cases. In total, 12,321 men (25%) had vasectomies."
"Vasectomy was associated with a small increased risk of prostate cancer overall (RR, 1.10 ...).
"Risk was elevated for high-grade (Gleason score 8 to 10; RR, 1.22 ...)
"and lethal disease (death or distant metastasis; RR, 1.19 ...).
"Among a subcohort of men receiving regular prostate-specific antigen screening, the association with lethal cancer was stronger (RR, 1.56 ...).
"Vasectomy was not associated with the risk of low-grade or localized disease."
Hard to ignore.
Freedland, in a paper this month :
"Herein, we detail the controversy between vasectomy and prostate cancer risk, particularly aggressive prostate cancer, by shedding some light on the molecular pathways, potential risk factors, and suggested links for those considering vasectomy and medical professionals who perform it."
"While the exact biological mechanisms and molecular pathways through which vasectomy may influence PC risk are unclear, possible explanations have included anatomical changes in the testis, epididymis, and vas, including changes in the secretions of vas proteins. Interestingly, a study found that insulin growth factor (IGF)-1 and IGF binding protein-3 (IGF-BP3) levels in the seminal plasma were significantly lower after vasectomy. While IGF-1 has been significantly associated with increased PC risk in humans, IGF-BP3 has been associated with lower risk of PC. As such, the net result of lower IGF-1 and IGF-BP3 on PC risk is hard to assess. Moreover, it has been found that vasectomized men have decreased expression of semen TGFβ-1 and TGFβ-3 relative to non-vasectomized men. Given that TGFβ and has an inhibitory effect on PC tumor growth[22,23], lower TGFβ levels could contribute to higher PC risk. Furthermore, it has been suggested that changes in semen protein behavior (both up and downregulation) may influence PC proliferation. Interestingly, the increased proliferation effect was modeled in rats in a study which examined the effects of vasectomy on the prostate. Like the human study, they found that vasectomy increased cell proliferation in the prostate post-vasectomy. More studies in both animals and humans are required to better understand these findings.
"Given the potential plausibility that vasectomy may link with PC risk, it is noteworthy that indeed, some studies have found a link between vasectomy and increased PC risk. In their 1998 systematic review of five cohort and nine case-control studies, Bernal-Delgado et al reported a 23% increased risk of PC among men who reported having a vasectomy. In two independent retrospective and prospective studies, Giovannucci et al found that, in almost 25,000 men combined, vasectomy increased PC risk by 56–66%.[3,26] More recently, however, a study from the Health Professionals Follow-up Study (HPFS) examined nearly 50,000 men, over 6,000 of which were diagnosed with PC, with a 24-year follow-up timeframe and found that of the 25% of the men in their cohort who had had a vasectomy, there was a 10% increased risk of developing PC in general, but a 22% increased risk of developing high-grade PC. Among these studies, one interesting observation is the consistent decreasing magnitude of the increased risk of PC after vasectomy over time, potentially related to the increased PSA testing over time, which has leveled the field in terms of PC detection.
"While the above studies certainly suggest a link between vasectomy and PC risk, this does not imply vasectomy causes PC. Rather, an association is a place to start looking and suggests that something is linking vasectomy and PC – whether it is the vasectomy itself (casual), the greater screening that occurs among men who undergo vasectomy (detection bias), or some other shared factor between vasectomy and PC (explained by a confounder)."
In the new study :
"We examined the association between vasectomy and prostate cancer mortality among 363,726 men in the Cancer Prevention Study II (CPS-II) cohort, of whom 7,451 died as a result of prostate cancer during follow-up from 1982 to 2012. We also examined the association between vasectomy and prostate cancer incidence among 66,542 men in the CPS-II Nutrition Cohort, a subgroup of the CPS-II cohort, of whom 9,133 were diagnosed with prostate cancer during follow-up from 1992 to 2011."
"In the CPS-II cohort, vasectomy was not associated with prostate cancer mortality"
"In the CPS-II Nutrition Cohort, vasectomy was not associated with either overall prostate cancer incidence ... or high-grade prostate cancer incidence"
Given the size of the new study, some are describing the 2014 study as "small" - which it is not.
Freedland mentions "detection bias" as a possible confounder, but that can't possibly explain increased PCa mortality. One confounder that I thought of a while back is unprotected sex. Marriage itself has been associated with PCa. In 4 days time my wife & I will have been married 49 years. We have never had STDs, but there have been countless opportunities for bacteria transferal. Who knows?