When feels empty during vipassana meditation?

When command the mind to does something it was done what had commanded.

"So, bhikkhus any kind of form whatever, whether past, future or presently arisen, whether gross or subtle, whether in oneself or external, whether inferior or superior, whether far or near, must with right understanding how it is, be regarded thus: 'This is not mine, this is not I, this is not myself.'

"Any kind of feeling whatever...

"Any kind of perception whatever...

"Any kind of determination whatever...

"Any kind of consciousness whatever, whether past, future or presently arisen, whether gross or subtle, whether in oneself or external, whether inferior or superior, whether far or near must, with right understanding how it is, be regarded thus: 'This is not mine, this is not I, this is not my self.'

SN 22.59

As the Buddha taught, identification with the mind (along with the other aggregates) is a wrong view. The mind is impermanent (along with all else in Samsara), in flux. It is not a stable, controllable thing. The more you try to control it, the more identification you will cultivate with it. Try meditating and observing thoughts arising and falling in the mind. You will come to see that they are out of the control of any self and you will come to objectively accept them as just arising and ceasing thoughts.

If thought about more happy today next moment less happy.

"Monks, these eight worldly conditions spin after the world, and the world spins after these eight worldly conditions. Which eight? Gain, loss, status, disgrace, censure, praise, pleasure, & pain. These are the eight worldly conditions that spin after the world, and the world spins after these eight worldly conditions.

AN 8.6

Experiences are impermanent as the Buddha teaches. This is something the Buddha teaches to accept. And how does one accept it? This is through meditation (specifically vipassana), observing one's experiential reality to see it objectively as it is and ultimately let go to be wavered by it.

After sitting meditation feel empty.

"And how does a monk remain focused on feelings in & of themselves? There is the case where a monk, when feeling a painful feeling, discerns, 'I am feeling a painful feeling.' When feeling a pleasant feeling, he discerns, 'I am feeling a pleasant feeling.' When feeling a neither-painful-nor-pleasant feeling, he discerns, 'I am feeling a neither-painful-nor-pleasant feeling.'

"When feeling a painful feeling of the flesh, he discerns, 'I am feeling a painful feeling of the flesh.' When feeling a painful feeling not of the flesh, he discerns, 'I am feeling a painful feeling not of the flesh.' When feeling a pleasant feeling of the flesh, he discerns, 'I am feeling a pleasant feeling of the flesh.' When feeling a pleasant feeling not of the flesh, he discerns, 'I am feeling a pleasant feeling not of the flesh.' When feeling a neither-painful-nor-pleasant feeling of the flesh, he discerns, 'I am feeling a neither-painful-nor-pleasant feeling of the flesh.' When feeling a neither-painful-nor-pleasant feeling not of the flesh, he discerns, 'I am feeling a neither-painful-nor-pleasant feeling not of the flesh.'

"In this way he remains focused internally on feelings in & of themselves, or externally on feelings in & of themselves, or both internally & externally on feelings in & of themselves. Or he remains focused on the phenomenon of origination with regard to feelings, on the phenomenon of passing away with regard to feelings, or on the phenomenon of origination & passing away with regard to feelings. Or his mindfulness that 'There are feelings' is maintained to the extent of knowledge & remembrance. And he remains independent, unsustained by (not clinging to) anything in the world. This is how a monk remains focused on feelings in & of themselves.

MN 10

When you feel empty, meditate on that feeling, observing it as just a feeling. That's all it is anyways.

Is it mindfulness?where is the meditation,according to visuddhimagga?

Unfortunately, I am ill suited to quote a passage from the Visuddhimagga that can clarify this (as it is a vast technical text). What I can tell you from what I have learned is that mindfulness is when one is in the here and now, presently focusing on their current experience (which, again, is done in vipassana meditation).

I would recommend checking Yuttadhammo's booklet on how to meditate. Your questions should be answered once you start meditating after a while. I hope this all helps!

When command the mind to does something it was done what had commanded.

According to visuddhimagga, above sentence is Concentration Meditation (Samatha). Samatha is an important base for Vipassana.

If thought about more happy today next moment less happy. After sitting meditation feel empty. Is it mindfulness? where is the meditation,according to visuddhimagga?

There are 2 mindfulness meditations, Samatha and Vipassana. Both are important.

What you are talking about is a kind of Samatha Meditation. It is a tool for for case by case management. If we don't think about more happy today next moment less happy, we have uncountable moments or objects to think of instead immediately. Vipassana is the next step to stop all of those clinging thinking forever.

For Vipassana according to Visuddhimagga and mostly suttas in first ten suttas of Diganikaya, when the practitioner can manage the present unwholesome mind arising professionally by the hard meditation on Samatha, the practitioner will have the strong enough wisdom to analysis Nama and Rupa (mentality and physicality in the dependent origination) as elemental smallest moment which depending on various origin (over trillion times per second; See Patthana of the detail).

The practitioner will see again and again the past karma (Sankhara) which the only one important origin of the previous life when the practitioner analysis life as Nama and Rupa, especially analysis each moment as elemental smallest moment which depending on various origin after the concentration meditation in the past and future life by Pubbenivāsānussatiñāṇa — Knowledge of the recollection of former living places and Cutūpapātañāṇa. — Knowledge of the rebirth of diceased beings. (Someone can do this without Abhinnaha depending on the past wholesomeness collection).

This past karma seeing let the practitioner the real 8 ignorant, 3 craving, 3 clinging (craving again and again), 3 karma, and the real moment of the realities (Nama and Rupa).

Each life has only one important past karma which keeping life going on, the other karmas are only the supporting role. No doubt left whether "Do good, get good, do bad, get bad." And the practitioner can analysis everything as Nama and Rupa. From now on, the practitioner getting the state called Cula-Sotapanna (Pre-Stream-enterer).

All above steps are the powerful encouragement and inspiration for meditating Vipassana in next step from now on.

The three characteristics will appear from this state on according to KN.Patisambhidamagga and Visuddhimagga the end of Kankhavitaranavisuddhiniddesa. The practitioner will use this three characteristics to meditate Vipassana (SammasanaNana, UdayabbayaNana, and the other uppers) on each real moments, not conceptual moment.

This is sound weird or so very hard depending on the reader's skill. To understand above, please memorize Sutta. Tī. Subhasutta, BhramaJalasutta, MahaSatipatthanaSutta (all in Pali and skillful memorizing only). Thank you Pa-Auk Tawya Sayadaw who let me understand the pali as it is, even I has no any chance to learn with him face to face. Amazing teacher!

the buddhas discovery is that the mind is not-self and therefore cannot be commanded as one by you.