Okay, let's break this down:
First off, you missed 1989-90. Rookie Richter went 12-5-5 with a .904, while Vanbiesbrouck had a .500 record with an .887. (Bob Froese was below .500, with a lousy .873.) It's rather telling that Richter -- a rookie -- got more minutes in the playoffs than Vanbiesbrouck. Both goalies appear to have performed well vs. the Islanders in the first round, but Richter won all 3 of his starts. Vanbiesbrouck did win the only game vs. Washington in round two, to give him his credit, but neither goalie could have won that series for them.
Yes, and for the second season in a row he destroyed Vanbiesbrouck:
Richter: 21-13-7, .903
Vanbiesbrouck: 15-18-6, .891
1991-92 - ok but significantly outplayed by Vanbiesbrouck
Not "significantly" -- that word would apply to Richter over Vanbiesbrouck the previous season -- but this year, it appears Vanbiesbrouck was a bit better. Both had great records. It's a great Richter season.
This is that weird, non-playoff season sandwiched between two great team seasons. Yes, it appears that Vanbiesbrouck had the better season. I wouldn't say Richter was "poor" -- his stats are still above-average, just not anything special.
Yes, a great season and Richter had enormously better numbers than back-up, Glenn Healy.
This one does look pretty weak. It's hard to explain how Healy ended up with better stats than Richter, unless Richter got all the hard starts and Healy the "easy" ones, but in any case not a good Mike Richter season. However, he did perform way better than Healy vs. Quebec in the playoffs. Both struggled vs. Philly.
1995-96 - good but hurt for a huge chunk of the season
Well, he played 41 games and went 24-13-3 (.638), while the team went 17-14-11 (.563) without him in net. This is another great Richter season.
Rangers are .590 with Richter in net, and .333 with Healy in. 'Nuff said. The Rangers also had a surprising playoff run, with at Richter at .932 and playing every minute through three rounds. A fabulous Richter season.
Then, there is the 1994 playoffs. And the 1996 World Cup, at which Richter was probably the best player in the world for those couple of weeks.
So, in conclusion, I would say you're way exaggerating in painting Richter as wildly inconsistent in this 1989-1997 period. In fact, there is only one season where he was kind of poor overall, that being the short 1995 season (the fact that this was the shortened winter-1995-only season might, in itself, be a factor as to why he under-performed, but who knows?). There is one other season, 1992-93, where he was less than great, and even that season his numbers are above average, just nothing special. So, for eight seasons, we've got:
1 x poor
1 x just above-average
6 x fantastic
legendary performances in the '94 playoffs and '96 World Cup