It's pretty clearly unexpected and unintended behaviour!
On the contrary, it isn't very plausible that they didn't expect this behavior - even the most basic of testing would have revealed it. Thus, it is a conscious choice to allow it to happen.
In general, they'd probably say that putting your e-mail address in the clear in a publicly readable place (where spammers can collect it) is not a highly valued capability, and doing something else with @ acceptable.
I disagree on two grounds - firstly, basic testing wouldn't necessarily have revealed it - if they never put email addresses in vBulletin posts, they could easily have just not thought about testing for it.
Secondly, if it was a deliberate feature, it would have been called out