Since we don't have the the setup to compare and can't see how much the difference is, we can only make an educated guess. According to the information here, I believe the operating system running on 2009 MBP should be OS X 10.11 "El Capitan". On the other hand, MBP 2020 should be running Catalina, which is the latest for now.
There are some posts on the web (e.g., here, here, or here) which mention that the font smoothing on modern MacOs versions, especially Catalina has evolved so that the text looks much worse on displays which does not have dense pixels such as the retina screens or the 4K screens of sizes between 21 - 27.
If the problem is not related to the conversion of signal to VGA, using the solution stated in here should decrease the problem just a bit. I had the same problem with a 24" 1920x1200 HP monitor lately, and these settings helped to some extent.
Nevertheless, it looks like Apple has optimized its software and smoothing algorithms for the high resolution displays and therefore, MacOS Catalina seems to be not as forgiving as the older MacOS versions. On the other hand, currently, Windows and Linux does not have this problem and can do better with lower resolution screens as of now.
Hope this helps.