First, remember that VPPs don't get to change to just anything anytime the player wants, except at a significant cost. So first, be sure you understand the circumstances under which they can change the VPP (perhaps by studying spell books for several hours), and what they can change to (e.g., only fire spells, or maybe only spells they have in their spell books).
Second, outside of the rules, unless the GM and Player are both very experienced Hero System players, and know and trust each other (e.g., one of the people in my current group and I have been playing Hero together for 30 years), I don't recommend allowing on-the-fly spell creation. Have the player create a common set of spells that they can switch between and then the GM verifies that they work the way they expect them to for the game.