In VS2010, I have a .txt file in my solution under "Solution Items". (i.e. the file is attached to the solution, not any particular project within the solution.) When the .txt file is open, the spell check option for VAX is grayed out and thus cannot be selected. This behavior also occurs if the .txt file is under a project.
However, in VS2008 (SP1), the spell check option for VAX is enabled for .txt files.