I consider sub-vocalization to be what goes on when you read something out loud (as in, reading a story to a child), but without full movement and making noise. But still producing the signals for your mouth and throat. This would be suppressed by counting out loud regardless of how automatic is the counting. Counting is good precisely because it is automatic, something more difficult could interfere on the conscious thought level rather than on subvocalization level. edit: instead of counting you can try singing a song while reading something unrelated...