I'm trying to recover from a near-complete nervous breakdown.

When I awoke this morning I had mild anxious burning pains in my arms and a tense emotional knot in my stomach. I started by paying attention to the mild pain in my arms - really concentrating on it and feeling it flicker about. I was glad that it began to dissipate within about a minute.

I then set about the knot in my stomach gradually as the day progressed. I kept returning to the unpleasant sensations - really focussing on them and noticing them churn about. By lunchtime these too had essentially disappaeared! 

I'm now just left with a subtle feeling of dis-ease/dukkha in my stomach....

Is this Vipassana?

"Vipassana" means "clear seeing" or "special seeing", i.e., higher wisdom. 

Its only vipassana if you clearly see the cause & effect relationship of how equinimity of mind causes the dissipation of the stress. 

As for the dissipation of the stress itself, this is called "samatha" ("tranquility"). 

Kind regards & good work 

hell yes, it's vipassana, and excellent practice to boot.

on the larger scale, I can imagine (as a veteran of multiple nervous breakdowns myself), that you're also simply feeling your way back into a relationship of trust with your body and mind and emotions: reopening, little by little, to the full experience of this incarnation, and now on terms that are somewhat up-for-grabs and yet to be discovered. Your working bodily self is a whole new country and you are exploring it. In a way, it's like you went through the bardos and took rebirth here, as this gestalt, and you're discovering what this gestalt is, body and mind, heart and soul. It's a very auspicious rebirth, lol: in a human body, and immersed in the dharma, and committed to grokking it. There have been turtles surfacing for uncounted ages, trying to hit a single ring in the ocean of samsara, to get you to this specific situation. So nice job; so far so good, lol.

God bless you, my friend.