Its a simple marketing strategy.
Any customer who can show proof of purchase of the vBulletin 4 software can then purchase XenForo 1.0 for the price of 1 dollar.
Or something that has a similar effect.
The idea being that if you paid money for vB4 and it was not what you thought i was going to be... XenForo will give you their software for free. Because XenForo 1.0 ended up being what a lot of us wanted vBulletin 4 to be.
Sure you lose money on perspective customers dissatisfied with vb4... but in reality a lot of vb4 customers will find it difficult to move away from something they just dropped nearly 300 dollars into. So you might not have gotten those customers anyway.
IMO Right now the biggest target audience for xF1 is vB3 users who wanted a modern version of the software, but not the broken software that vB4 ended up being. Thats xF1, and so you will get a ton of those customers.
This idea allows you to also get a lot of the vB4 users you might not have gotten, plus it creates a ton of goodwill for XenForo LTD, as well as a huge marketing opportunity.
I expect admin/forum centered websites to run with this story and give xF1 a huge amount of good publicity.
Sure its a stunt, sure some may see it as taking a cheap swipe at Internet Brands... but at the end of the day its the consumer who is going to determine if its a cheap stunt or a valuable resource for those who already spent their money.