Dear A:

You wrote:

> Hi there.
>
> I contacted your website a while ago regarding how to address a former Vice
> President of the U.S. You were very helpful then and I hope you can help
> me now.
>
> I argue that the following is correct:
>
> 1) “It is she who will be involved in the major event.”
> OR
> 2) “This is she.” (as one would respond on the phone when asked for by name)
>
> A staff member argues differently (against this). They seem to think we
> should use “her” in these situations.
>
> WHO IS CORRECT?…. if either. Thanks.
>
The grammatically correct versions in both cases are those written here. “It is she who will be…” and “This is she.”

In both cases the position calls for the nominative case pronoun since it is a predicate nominative. Having said that, in everyday speech, both in the US and the UK, people do commonly say “This is her” without any ambiguity.
If I were quoting a conversation, I would not change it. But if I was in a situation where my grammar might make a difference in how I am perceived, I would say “This is she” or “It is she.”