MyBB is good however for themes u have to buy OR pirate OR have someone code for u. MyBB very simple to install /configure /use /administrate. However it does lacks the eye candy.
There are plenty of MyBB premium themes available. Some you can get nulled, some you have to buy. Usually they charge 50~100 USD for a customised theme which suits your forum niche (which is very important)
On the other hand I like xF, which is not powerful as IPB and lacks some essential plugins like market (though they have one now) etc it is the best good looking forum software IMO. don't worry about Internet Bulletin law suit and all, they are going to stay.
However xF is costly in long run, customised themes are costly, developers for plugins are costly & plugins also ! In MyBB many plugins are free and very few are premium. But most of the free ones does the job, you hardly ever require to go for paid plugins.
In xF, not many plugins are free & the no. of plugins compared to MyBB is far less. Also very less (2-3) free themes are good in xF unlike MyBB.
I don't know why you would call vanilla as crappy. It is best at what it does. It is not powerful though.
Regarding nulled scripts, you can use it. Unless someone reports you, chances of getting caught is less. However if you are new, then u need lots of support in beginning so better buy the license.
I first used xF nulled, I liked it so much I bought a copy because I wanted to support them and also I felt they deserve it. xF doesn't cost a bomb, try a nulled first or their official demo, if you like it then buy it.
TLDR,
MyBB : free, powerful, easy to customise, lacks eye candy
xF : costly, not so powerful, lacks some essential plugins but looks kickass.