Can you escape a % sign in the write file step?
 

so that it will actually print out instead of VBP expecting macro content.

Example %a%b%c%4d

Right now VBP errors and says "a is not a valid macro".

The percent sign character % is normally used around a macro name for its value to be expanded within the field; to insert a literal percent character, use two percent characters %%.

http://www.visualbuild.com/Manual/stepproperties.htm