When feels empty during vipassana meditation? When command the mind to does something it was done what had commanded. 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?
When command the mind to does something it was done what had commanded.

&quot;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.'
&quot;Any kind of feeling whatever...
&quot;Any kind of perception whatever...
&quot;Any kind of determination whatever...
&quot;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.

&quot;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, &amp; 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.

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