Shinseki may or may not bear responsibility, wholly or in part, for the situation. That's the problem with calling for his resignation: we don't know if he's responsible. What good will it do to force Shinseki out if he's not part of the problem?
In fact, the problems ar the VA are so widespread that I can only see two possible underlying reasons for them: either a systemic culture of corruption and lying, or a shortfall of resources.
If the culture is rotten, Shinseki should be the first (but not the last) to go.
However, if the VA simply hasn't been given the resources it needs, the problem isn't with Shinseki. It's with Congress and its unwillingness to fund government services at proper levels. Funny how we don't hear Congress talking about that.
Fairness dictates we wait for the VA Inspector General's report. However, since some members of Congress have seen fit to demand action without knowing what's going on, I will, too.
I don't believe the VA's culture is that rotten. It's a lot easier to believe that the VA is underfunded and understaffed as a direct result of Congress' refusal to do its job.