is the pid of the fork, and not that of wvdial itself.

That got my attention. Then I noticed you're using system( "wvdial" ); instead of exec( "wvdial" );. Try this with exec().

system() launches a process and waits for it to finish (so it's appropriate for wget).

fork() + exec() launches a process that runs in parallel (the appropriate choice for wvdial).