Ok. Seems like 3 big things to consider. #1 “Vintage” suspension and brakes will beat you up more but my naivete tells me maybe that can be negated somewhat by conditioning and the idea that i have not been spoiled by modern technolgy.
#2 Wheels and tires 26 inch clinchers being less efficient/cushy than 29 inch tubless.
#3 The shorter wheelbase of the modern MTB.
What about the geometry has changed besides wheelbase? If you lock out the suspension, how have ST and HT angles, chainstay length, trail, etc changed. And of the vintage MTBs, which ones were ahead of there time in this regard.
Please pardon my ignorance. My only experience on a MTB is two C class races on a turn of the century Rockhopper.