A better link for a beginner would be
http://cshandley.co.uk/JasonHulance which is also in AmigaGuide format with the main AmigaE download. To answer the question more directly, you need to include the module called 'dos/dos.m' to allow direct access to DOS.library functions. VfPrintF is the equivalent of PrintF but to a file handle. If you need to compile a Kickstart 1.3 compatible executable, pass the file handle, the EString itself, and the length of the EString to Fwrite.
Note that system functions must start with a capital letter followed by a lowercase letter. If you can't figure out the correct case sensitive spelling, use showmodule on the library's module file.