Air carrier pilot on a vector to join the final approach course reported a temporary loss of control due to encountering severe downdraft and turbulence.

Narrative

On the ZZZZZ arrival into ZZZ there was a line of dissipating convective activity (virga; reported moderate turbulence; green radar returns turb) extending from the east of the airport towards the NW. We requested right of the arrival course to avoid the worst of it. We were set up for the RNP Z XXR and configured to Flaps 5 degree; 180KIAS; AP/AT on with continuous moderate turbulence. As we neared the final course we were too high due to the deviation and traffic and had to be turned north towards the virga; 360 degree heading; 5;000 ft. Other aircraft were transiting the area and just reporting moderate turbulence so we felt comfortable with the vector. ATC gave us a right approximately 200 degree heading to join final and descent to 3;000 ft. Once established on final; in LNAV; outside of ZZZZZ1; ATC instructed us to maintain 2500 ft until ZZZZZ1; cleared approach. Passing approximately 2;800 ft.; stable at 180 Knots in V/S approx 800-1000FPM. I looked up to select VNAV and set field elevation. Almost immediately we hit a jolt of severe turbulence; and the FO emphatically says 'AIRSPEED!' The second it took to look away from the PFD (Primary Flight Display) and look back; nothing now appeared the way it did before. We had an instantaneous 30 Knot airspeed loss continuing to trend down; large downward trend on the green deceleration indicator; were close to or possibly in the lower yellow band; had instant 300 ft. altitude loss and VNAV disconnect. I immediately disconnected the AP and lowered the pitch attitude; due to the turbulence it took me a second to get my right back on the thrust levers; disconnect the at and advance the power levers and recover from the UAS (undesired aircraft state). The aircraft was initially uncontrollable and felt mushy due to negative energy state. We momentarily descended below 2;500 ft. started accelerating towards 180 knots as the engines spooled up. Once clear of the area were able to continue configuring make a normal landing. We reported the event to ATC; Dispatch; FODO (Flight Operations Duty Officer) and wrote up the aircraft in the maintenance log. Some takeaways I have from this event. I suspect we encountered a downdraft from the dissipating cell. Second; it reminded me of high altitude upset training we did a few years ago in the simulator where you are flying along and all the sudden you encounter an upset and nothing is as it was just seconds earlier. Finally; our A FA has been here 37 years and said that it was worst turbulence he had ever encountered. They had been briefed and were seated from TOD. Just because other aircraft are proceeding through an area of dissipating convective activity doesn't mean it's safe to do so. Next time I would request vectors away from the Virga.