Trying to spin the index news now patrick?
Fund managers won't have to lighten up on Zen because that haven't even bought Zen yet. They're required to do so on Monday, 4/22/13.
The reason for the effective date of April 22, 2013 is to provide investors the time to accumulate before the ETF is required to buy. Index funds cannot make changes to thier portfolios until the index is officialy changed.
And coming back to you question, well no. It's all about Zen's market cap which will determine how much they are required to buy. If all the VSX stocks in the index are tanking and Zen is level or going up, well then Zen will have even a greater market cap % and they will buy more.
Even so, I don't believe the index will effect SP all that much, besides a small pop on Monday. However, it will increase our exposure and possible lead to additional analyst coverage which may have even a greater impact.