I haven't been vintage racing in some years now here in the Pacific Northwest (so it may be different now...) but it was/is a hard and fast rule that if it wasn't built before then it's not vintage and it's not coming on the track for a vintage race.
Personally, I thought this bright line approach was/is really good. It keeps things respectful between the drivers who are driving million dollar vintage cars, with all the short comings that comes with that....and those who have a lot less invested in their modern recreations.
It also provides some interesting contrasts keeping everything vintage and open...[[[.seeing an original 1968 GT40 overtaking a 1965 Hillman Imp is pretty entertaining.]]]