Both sentences are grammatical; the difference between the two is in what the speaker's feelings are about Victoria's wants.
What do you want, Victoria?
What is it you want, Victoria?
"What do you want?" is emotionally neutral.
"What is it you want" uses 'it' to indicate that Victoria's want (it) is separate from Victoria.
The two sentence variations have similar but not the exact same meaning. The first refers to Victoria's want as a property of Victoria; in this case an emotion that is part of Victoria. The second refers to Victoria's want as something separate from Victoria but which Victoria possesses; it implies that her emotion is not a property of Victoria but rather something she possesses as though she were carrying the emotion around in her pocket.
Disassociating a property of a person from that person is sometimes used as a method of convincing a listener that the property is not really part of their nature; often as a way of getting the listener to remove that property from themselves. In the example sentences, the use of "what is it you want" implies that the speaker is hostile to Victoria's want and wishes her to stop feeling that way, while "what do you want" is more neutral; the speaker is not indicating how he feels about Victoria's want.
In normal English usage asking someone what they want at the start of a conversation implies that the speaker is being interrupted and so is less than polite. Using 'what do you want' as part of an ongoing conversation does not imply an interruption and so is less impolite.