To ensure a proper flush, just push the handle all the way down and release. While anything else may initiate a reasonable flush, it is not designed in or intended. Now, since nearly all new toilets do not use the entire tank's worth of water, if for some reason, you feel the need to use more water, you can just hold the handle down until the tank is empty. This is 99x out of a hundred a total waste of water, as the thing works with the designed 'normal' flush volume. If you try to short-circuit this with a partial push, you may likely end up using more water, since you'll need to flush again. IOW, use it as intended.
Toto does sell dual-flush toilets that produce repeatable, measured flush volumes that do their intended job, they have a two button activation, making it straightforward and reliable - the Vespin isn't one of them...