I don't know if you can record on the same track.  Have you tried it?  If it doesn't work. you can always hit Undo (CTRL+Z).

Still, you can always record on the next open track and then move the recorded audio object up to the track you want it in.

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Yes, you can manipulate note velocity after the fact in Vita.  That's the beauty of MIDI.

To open the MIDI Editor, either double-click on the MIDI object or go to the object menu and click on "MIDI Editor" (CTRL+SHIFT+D)

Make sure the Velocity/Controller Editor is open by clicking the "graph" toggle button at the top of the window.

At the bottom of the MIDI Editor, you will see the Velocity/Controller Editor (the default setting is velocity).

The note velocities are designated by corresponding vertical bars.  If two (or more) notes start simultaneously, the bars will be superimposed on one another and their individual levels marked by a horizontal line.  The velocity of the notes is also designated by their color: the darker the color, the harder the hit.

There are two ways to manipulate the velocity in Vita.

To change velocity values in the Velocity/Controller Editor, you must be using the "manipulation" tool (the button on the left side with an ARROW).  In the Velocity/Controller Editor, you simply hover over the top end of any vertical bar, then click and drag it up or down, then let go.

You can also manipulate the velocity of any note (horizontal bars). Click on the Velocity tool button at the top of the Vita window (see first screen shot) and hover over the top edge of any note until an up/down arrows appear.  Then, click and drag it up or down, then let go.  Using this method, the note will also sound as you change the velocity.  This is the easiest method to use when there are multiple notes that start at the same time.