mmm...sounds like a last modification date column for prices will be needed, like the one for products.
But they are already here in the table #_virtuemart_product_prices: created_on,created_by,modified_on,modified_by
So we could make this column appear on the product list with something like "Since: + modified_on date of the price".
And a parameter in backend could prevent the modification of the price if date() - modified_on !>= one month (particularly in case of discount)
...just a suggestion