Found a very interesting YouTube comment that makes a ton of sense - why I used to be able to play New Vegas but can’t anymore.

A 32-bit app on Windows is almost always limited to 2 GB (2,048 MB) of memory for both itself and any libraries (aka “.DLL files”) it needs to run, and the memory taken up by those libraries is not shown in process manager. This was adequate at the time FNV was released, but in the intervening dozen plus years, the memory required by these libraries has grown quite a lot. On Windows 10, they take up nearly half of the memory available, which is why FNV crashes when it has reached about 1,000 MB, and needs to allocate more.

So even though you have plenty of ram, the 2GB cap has to be shared by the game along with the much larger processes that windows 10/11 use compared to Windows XP/7, which are needed to run the game.