I agree with NSR. Half the fun of Vintage Enduros is checkpoints-timecards-special tests etc.... That's why it's Vinduro not Vintrailride....
But seriously - there's no reason that you have to race in a Vintage (timed) Enduro - just have a Rally (non-competitive) class as well as the normal classes. Even without the Rally class, if you do away with the 'houring out' rule, there's no reason that anyone has to treat it as anything other than a trailride anyway. Do a lap when you want, don't take a time card etc... When you get to a checkpoint, if you aren't interested in the competitive side of it, just ride slowly through and laugh at the guys waiting for their minute to come up on the bundy clock!
And as for classes, I think age of bike is more important than capacity.. maybe you break it down to under 250 and over 250, but you probably don't need to go any further than that.
I think we had a good long talk about this a year ago. Pre 75, Pre 78, Pre 81, Pre 85 or similar?