Changing the font in the ToC?

The commands for the TOC entries are different to the commands for the headings. To get sans serif fonts in the TOC, redefine the \cftXfont commands, where X stands for chapter, section etc.

This will achieve sans serif chapter and section entries in the TOC:

\usepackage{tocloft}
\renewcommand{\cftchapfont}{\normalfont\sffamily}   
\renewcommand{\cftsecfont}{\normalfont\sffamily}   

LaTeX tells you that it is find a font. It also tells you which font it uses instead:

 LaTeX Font Warning: Font shape `T1/lmss/bx/sc' undefined
 (Font)              using `T1/lmss/bx/n' instead on input line 8.

Substituations are normal. As you can see here a sans serif font is used as substitute.

\chapnamefont defines the font of the vertical chapter beside the large "2" not the font in the toc.

For the problem with "viii": http://www.tex.ac.uk/cgi-bin/texfaq2html?label=tocloft

To make the TOC sans serif, just add the command \textsf, like this: \textsf{\tableofcontents}}