I show up at group rides all the time with vintage bikes from the 6 speed, downtube shifter, friction shifting ( eroica compliant) era. The reaction ranges from ambivalent to curious to irritated. Some people get irritated because they have bought into marketing drivel that they can’t possibly keep up with the group without the latest load of whatever is being sold at the moment. For efficiency in moving down the road, the reality is that bikes have been good for a long long time and there is very little…if a any difference between a 1975 Campy equipped DeRosa and a 2022 Dura Ace equipped Trek Domane. Aero wheels will obviously be an advantage when solo or in the front of the group, but as speeds fall off from 25 mph or so, those advantages evaporate. For spirited group riding, I don’t think that a nice light set of 32 spoke tubular wheels has ever been surpassed. Where you will find yourself at a disadvantage on a vintage bike is when the ride includes lost of choppy hills with frequent accelerating. With downtube shifters, you can’t shift while out of the saddle and you have to plan your shifts a little more. Still, most of this effect can be mitigated with practice and mastery of your equipment.