1. XenForo Future.
Xenforo is still on rocky grounds and the company is struggling atm due legal issues. Xenforo is a non stable company atm and to pay for such a software would be unadvisable and a cost to the community if the company is shut down. Something that is still a possibility.
2. XenForo A Step Back.
The current software provided by Xenforo would be a technical back step and the features provided ether a resource killer or just not needed and many of key selling points fail to make the software valid. With this also said the Xenforo would cause issues to many members with its coding being very heavy.
3. Not The Best Alternative.
Why no one wants to admit big companies make better products the fact remains that a company that can place 10000s of man hours into a piece of software will normally out weigh one that can only put in 100. Currently the only viable alternative to vB would be IPB [IPS].
4. Features + Support
No matter how any one tries to hurt vB no one can argue the fact the amount of features and support offered by vB is second to none.
5. Mods + Addons
Again vB Is the leader for mods and addons created by the vB community. Any thing you want on your site is either already built and working for you to use or there base structure is there for you to expand on. The great thing is most of these apps are free.
Overall,
IPB is the only alternative to vB that could be classed as viable. A move to Xenforo would be unwise and could possible cause issues to the community at a later date.

