SR22T Pilot reported observing a Cessna that turned and descended on a converging course resulted in a NMAC.

Narrative

I was IFR on a heading and altitude with ATC being vectored to an approach at ZZZ. There was a VFR aircraft to my right; 500 ft. above me; a reasonable distance away. ATC informed me of the traffic; I spotted them; no issue. VFR with 10SM viz.Then suddenly; they turned towards me and started descending. Before I could even get ATC on the radio they're maybe 200-300 ft. above me. Dangerously close. I had them visually the whole time. There is absolutely zero chance they didn't see me; clear as day and my plane is colorful. Thankfully I'm in a Cirrus and they were in a slower Cessna. I was on their left; the PIC must have been paying absolutely no attention to traffic. I realized after the fact I should have just made an immediate descent to provide more separation; but clearly they were going to overtake me and I had nowhere to go except nose down. They put me in an extremely dangerous position at only 3000 ft. Hindsight; of course; but I'm just absolutely shocked they did what they did. I believe this happened between XA15 and XA45 local time. Definitely between ZZZZZ intersection and ZZZ; probably around the ZZZZZ VOR. The person at fault is the VFR plane who began their descent into me. They were on my ADS-B but perhaps they don't have ADS-B in. In a busy area such as ZZZ; one must keep their head on a swivel at all times. I regret not descending immediately but; again; I was already at a low altitude and kept them in my sights while maintaining 3000 ft. and my heading (I believe was 180). Looking back I should have called ATC to report it but ZZZ Approach is always task saturated and I was still in shock. I've definitely learned to trust no one in the air; never assume they see you even though they should. I'm hoping if it was a flight school plane; this is a valuable lesson for the student and CFI (though they didn't notice me). It certainly was for me.