Arctic Fox browser. I can say it runs decently well on my Vista VM (although it's slow at times due to the low amount of RAM I have allocated to my VM and my old processor, in my case a Core Duo T2500). I found it that renders the sites I have visited without any errors (keep in mind that I just started using it yesterday so as of now I haven't found any bugs yet). As for slowness, the browser isn't exactly slow, its my VM being slow. The browser should run much better on yours provided that you have a physical machine running Vista instead of a VM, which should run way better compared to my virtual machine.
Link to download the browser: http://o.rthost.win/gpc/files1.rt/arcticfox-27.11.0.win32-git-20201121.7z
(Do note that you don't need to install it. Just extract the 7z file with your preferred zip file archiver, open the extracted folder, click on "arcticfox.exe". A pop-up message will appear, just click 'Run'. There may be another pop-up window telling that Windows Firewall has blocked the browser, just click on 'Unblock' and you're all good to go!)