This differences between a vintage bike, and upgraded vintage bike, and a new bike are a bunch of little things that all add up. Going through that process shows you exactly what those things are and what the effect is. Sure, you can hang with similarly fit people with new bikes on a group ride. But you'll be working a little harder, a little less comfortably.
I didn't used to believe that, because I was riding vintage bikes when they were new...I got away from it for a while, then came back to what is now called C&V, but in recent years I learned by upgrading and finally buying a new bike.