Word counter for text files
Usage of interfaces

HashMap&lt;String, Integer&gt; dictionary = ...


Try to use interfaces on the lefthand side as a best practice: Map, List,  ... as it will make you independent of hashmap, linkedhashmap,...

also try not to use vector unless really necessary (https://stackoverflow.com/questions/2986296/what-are-the-differences-between-arraylist-and-vector)

Naming

HashMap&lt;String, Integer&gt; dictionary = new HashMap&lt;String, Integer&gt;(); 
...
Dictionary reader = new Dictionary(dictionary);


There is a class dictionary, a map with the same name and when initalizing an instance of Dictionary, then it is called "reader". This is confusing

reader.fileScanner(text)


This method name does not really indicate an action. reader.scanFile(text) would be better.

Reusing dictionary data

you could make the counting of the words a method of the dictionary where you build a new map with only a count for the words encountered in the new text file(s) and return that map.

Comments

avoid comments like 

// Assigns the value of the parameter to the field of the same name


as they describe what you do in the code, which you can see by reading the code. When putting a comment then at least tell why some piece of code is implemented as such (specific issue, specific algorithm, ...)

Error handling

e.printstacktrace is by preference replaced by a logging framework (log4j2, logback, slf4j as façade, ...)
Perhaps you can make the method throw ioException and make the calling code handle this?