We're having a discussion in here which you might like the wavelength of, since this might end up meeting your requirements...
http://forum.virtuemart.net/index.php?topic=47990.0Food for thought?

is this just a discounted price that you revert to the normal price after the promotion or is this an item that you only have during the promotion?
If this a normal product that is placed on special VM is already setup to do time based discounts just select Product Discount List in the product menu and add a discount it is time based with a start and end date. I have a whole years worth of discounts already setup then all I have to do is select my preset discount from the drop down list in the right hand side of the product information field.
I hope this helps you out if not maybe your specific request will be implimented into the next version
Hey scottaaws,
Just noticed this post, and yes, I totally agree, if it's a timed based discount, the Product Discount feature should do the job nicely. I was thinking more along the lines of setting the product in the backend to show up and disappear within a certain time period, using the "Availability" as your start time and a modified version of it, "Expiry Date", to mark the end time.
Check out the link I posted above for more info...
