This was a bit confusing to me as well the first time I did it

. Maybe they could make this a bit more 'user friendly' ?
To
buy the blog, go into members.vbulletin.com
Find the license you want to buy the blog (or project tools) for
Click on upgrade/renew
@ the bottom of that page, you will see Project Tools and Blog
Purchase the quantity (1) you want.
Checkout
The license is assigned to the proper forum.