I can't brain real good right now. The VP increase definitely needs a clause stipulating a minimum. I think a floor of 0 is a must and I'm inclined to consider a minimum of 1, or maybe 1 every other level.
This is why you have the option to take the average instead of rolling. It's an issue that also exists in the standard HP rules. Jeremy Crawford covers those here, if you want to check them out:
https://twitter.com/JeremyECrawford/status/938885709783384064?s=19
That said, I see no problem...