Windows 7 didn't use less resources than Vista, it was virtually the same, it's just that PCs have gotten miles faster in the timespan between Vista and 7 and people thought it was the OS that improved.
7 was not that much different from Vista, the problem was that When Vista launched, most people still had Windows 9/XP level hardware and that's what most PC retailers were still selling as well, while Vista demanded more as it was a complete overhaul from Windows XP/9.
7 didn't improve much performance wise, it's just that by that time people had already upgraded to much better hardware and they felt a performance improvement.
By most metrics, 7 was basically a Vista Service Pack, renamed to distance itself from the bad press Vista got, but it was more or less Vista under the hood.
For me that made a huge difference, with Vista I had to always stop the search service
But still... assuming a decent non-launchdate machine (1GB RAM, Geforce 6200), Windows 7 was more performant overall on the same hardware, and this was never 'fixed' on Vista's codebase over time. You can pretty much notice the small but important optimizations in the search service (faster, less lag), resizing explorer windows (less redraws), less scheduled services running, user account control alerts weren't as obtrusive. Sure, you wouldn't get extra fps on your favorite game, but those quality of life improvements really did show on daily usage.
After 7, I switched to Mac, and even after all these years, I still have fond memories of that OS. Every time I tried a new Windows version, they seemed to have gone downhill.