Although when I checked at vBulletin site, they do present one "coupon code" field. Do those(IP/VB) sometimes give discounts when purchasing?
They may do some for events such as Easter, you'd have to check their site though. If they have any, it'll be on there.
I have seen only the "starting price" offers for vB - like the mobile suite discount, but no discount codes as such.
VB rarely gives discounts. The only discounts I know they give is for buying multiple licences at once.
IPB gives you a discount if you buy multiple products at once - I think it's 5% per product, cumulative over the whole order.