How much time will a candidate spend answering questions? For me the best evidence of capability isn't in a spoken interview, I would rather want to look at code they produce to achieve specific tasks. I would make it specific to the types of code the individual would be expected to write (e.g. if ADO is a requirement, then make them complete a small ADO task).
Ask the candidate to write a specific function (UDF) that will reverse the text in a string. Or something more complicated if you like... Maybe you can challenge their numeracy skills in this exercise?
Ask the candidate to write a function that will hook up to a DB and return a recordset according to a given SQL command.
Ask the candidate to develop a userform on the fly...
The skies the limit. Many people have read VBA books and can speak competently and confidently and are they can be quite convincing. Many struggle with the application, and at the end of the day it's the application that counts. I think the time is better put to use by having them undertake a few exercises... You can learn a lot about an individual by reading their code... Things to look for:
Do they apply naming conventions?
Do they know the Excel Object Model well enough to use it's native functionality to solve problems rather than using bulky code (e.g. deleting rows using AutoFilter rather than backward loop)?
Do they choose to load arrays and then work with the array data rather than looping through cells?
Look out for bad habits like unnecessary use of "Select" and "Activate".
Are they making proper use of variables, and are they typed explicitly?
The list goes on, but my point is that a little code reveals a lot more than a verbal Q&A...
Good luck, I hope you find the right candidate(s)!