I don't know if IBM created the AS/400 as a VAX killer. I doubt it.
The AS/400 is a direct descendent of the S/38. The S/38 was designed
and built by the people who brought you the S/36, S/34, and earlier
machines. The IBM plant that built those machines and related equipment
has been in Rochester MN since the late 1950s.
Both series of machines competed in the mid-range market. I don't think
that IBM has 'won' the mid-range market with the AS/400 and iSeries. I
think it is just the last player standing in that market, with Unix
machines having done serious damage to all of the players.
Mike