hello everyone i love to participate in this discussion as i love different types of script i can make you understand regarding the merits and demerits of the forum script state above.
Vbulletin is not only the expensive paid forum script but also there are lots of good and highly expensive forum script available in the market among them " invision script " also one of the most expensive script, I was purchased this script for my client since last 6 to 7 months with the cost of 250 dollars with a cms module and it has many more feature than Vbulletin.

and some people have some wrong concept that SMF is a free script and also not good but i want to make you clear that, its also paid and its cost is $ 125 per year if you purchase the licensed version you will get much more facility than Vbulletin and for this you need to contact the smf support team with E-Mail and those people using it its not a licensed version its the free version. previously Vbulletin was available for free and many people downloaded different version and still usingit in their site and telling its a paid one and they invested much money. I am having 3 different version of Vbulletin with Premium quality themes and modules and the demerits in vbulletin is -if u r a layman then you can't manage a vbullein forum because its internal management system is very tough you can't add and edit anything in the script and you need to generate a license key and they need to add it in software before installing it and then it will work otherwise it won't work and its security is high but its many time hacked by the advance hacker with simple 2 to 3 lines of comment. I am having it but don't tell the procedure otherwise people may put other vbulletin forum owners. So these are the merits and demerit of the forum scripts.
Thanks.