Given the reissue nature of the the model hobby, I think it's hard (or futile) to try and pick a specific year or dividing line between what's a vintage kit and what's a modern kit. Within a given year, model companies have a variety of straight reissue, modified reissue, and new tool kits available.
There are some significant years, though, like 1988 when MPC's last annuals were made (they continued in 1989 as AMT reboxes)...and also in the mid '80s when Revell and Monogram became Revell-Monogram. Or 1978 when AMT's last annuals came out (though they sporadically have had some later). Or 1979 when the last new tooling came from Jo-Han.