I do not find any Vault API that will allow you to control the view or zoom of the model in Vault Explorer. (on the Preview tab)
If this is important to you please make a request that this be supported from the Vault API on the Vault Idea Station. (Or to have settings in Vault Explorer to control this)
You could use the Inventor View Control from a Vault Exporer AddIn to view Inventor ipt or iam files. This blog post has some training that uses the Inventor View control.
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