If you're calling the third party process from a Run Program step, choose 'A file' on the Program tab and enter the filename there.
Or
1) Use a script expression like this in a Log Message action:
[vbld_GetFileContents("c:\path\to\file.ext")]
or
[vbld_GetFileContents("%FILE_TO_READ%")]
(if the filename is in macro). Demonstrated in the XML.bld, Advanced.bld, and Script.bld samples.
http://www.visualbuild.com/Manual/scriptexpressions.htm
http://www.visualbuild.com/Manual/sysscriptfile.htm
2) From a Run Script action:
Builder.LogMessage vbld_GetFileContents("c:\path\to\file.ext")
3) From a Run Program action:
%DOSCMD% type "c:\path\to\file.ext"