What is the difference between access and absorption in Vipassana meditation?
This is more an unfortunate example of problematic translation than anything. appento, for example, means rushing forward, plunging or entering into, or fixing upon. The commentary surrounding this word, 


  sampayuttadhamme ārammaṇe appento viya pavattatīti vitakko appanā


would better be translated as 


  "Applied thought that proceeds as though plunging into the object with its associated states is called appanā"


vyappanā could mean several things, since it is just appanā with a vi prefix. You'd have to be fluent in Pali colloquialism to understand the meaning, but PED says 


  Vyappanā (f.) [vi+appanā] application (of mind), focussing (of attention) Dhs 7. 


At any rate, "absorption" should be understood in the sense of "becoming absorbed in (i.e. fixed upon) the object", I think. 

"Access" is another ambiguous term; it is used in the sense of moving (cara) close to (upa), as it is a translation of the term upacāra. Other translations are "neighbourhoood", "approach", etc. It is more often used in a mundane sense to refer to the area surrounding (e.g.) a dwelling. e.g.:


  vihārassa upacāre - in the surrounding area of a dwelling (Vin. Pāc. 2.5)


In regards the section of the Visuddhimagga you are reading, we should first be clear that this section pertains to samatha meditation, not vipassana. The two terms are not commonly used in the practice of insight meditation, since it is understood that appanā samadhi doesn't allow for insight to occur. Insight meditation requires momentary concentration similar to that described as upacāra in this section, in that it comes close to but doesn't reach samatha jhāna.

The main difference between the two is that appanā is fixed on the object from moment to moment, unwavering and unchanging - it is sometimes described as "trance" concentration; whereas upacāra is still not fixed on a single object. The main similarity is that they both have the power to suppress the five hindrances from the mind.