Código PHP:
function vista(){
document.form.action="previa.php";
document.form.submit();
}
<form action="enviar.php" method="post" name="form" id="enviar">
<input name="submit" type='submit' id="submit" value="Enviar">
<input name="reset" type='reset' id="reset" value="Borrar">
<input name="button" type='button' id="button" value="Vista Previa" onclick=vista()>
</form>
