Sperm re-absorption is not something that happens solely to vasectomised men - all men produce millions of sperm that do not make it to ejaculation and are reabsorbed into the body. Sperm within the testicle are immature and cannot fertilise an egg - they mature in the epididymis.
Referring to the diagrams below, basically what happens is the sperm are moved from the seminiferous tubules within the testicular structure to the epididymus for maturing and storing. They stay in the epididymus for about a month, and at the end of this time are either degenerated and absorbed, or are expelled to begin the journey up the vas deferens to the ampulla for storage prior to ejaculation. Animal studies estimate that 40%–50% of sperm produced are reabsorbed before they reach the vas deferens.
When some obstruction to sperm exiting the epididymus occurs, a "Compensatory action" happens to avoid permanent testicular dysfunction. Obstruction does not necessarily mean vasectomy - various medical conditions may cause temporary or permanent obstruction.
In the case of vasectomy, initially, the diameter of the ducts increases 2 to 4 times their original size to counteract the increase in fluid pressure, and fluid absorption is increased, particularly in the efferent ducts, where 90% of excess fluid is reabsorbed. This, and other changes seem to delay significant problems, but due to the ongoing production of sperm, further alterations often take place to avoid permanent testicular dysfunction.
Post-vasectomy, the level of macrophages will increase within the epididymus. Macrophage means "big eater". Macrophages are white blood cells that crawl around in the extracellular fluids of your body and gobble up microbes and other foreign material. They ingest these microbes by phagocytosis ("cell eating"). Parts of the cell surround the particle to be eaten, then the macrophage's membrane flows together and the particle ends up inside.
3 months after vasectomy the number of macrophages laden with phagocytized sperm had drastically increased throughout the epididymis. the task of reabsorbing the sperm over-whelms the epithelial cells (the usual method of re-absorption) and more macrophages are recruited from the circulation to aid in their digestion and clearance.
Testicle and Epididymus
To explain the diagrams
The epididymis is an extensively coiled tubular organ attached to the posterior surface of the testis. If you uncoiled it, you would end up with one tube about 5-6 m (about 17 ft) long! Cross-section 1 and Cross section 2 are of the epididymus. Although the pictures give the impression you are looking at several epidydimal ducts, what you see a cross section of one highly convoluted duct. Cross section 2 is a magnification of Cross section 1.
The duct is lined with tall columnar epithelium with prominant Stereocilia.
Basically, this is a sheet-like lining entirely devoid of blood vessels, but rich in nerve endings. In the epididymus the epithelium is "columnar". This means it's not a flat surface, but highly textured and looks a little like the texture or a brush under magnification. This extends dramatically the surface area available. Cross section 2 shows the brush like texture of the inner surface of the duct.
The stereocilia are actually extremely long microvilli (finger-like extensions) covering the free surface of the epithelial cells. As in the columnar structure of the epithelium, this allows a tremendous increase in surface area of the cell for absorption or secretion. Stereocilia are not able to move in the powerful manner characteristic of true cilia. 'Stereo-' means firmly attached or in other words non-motile. They occur only in two places - here in the epididymal duct, and in the inner ear.