We tried many bridges into other forums and they simply were no good, we only found one other forum script that integrated with WHMCS and that was pretty poor.... Then we found WHMCSBB ... Simple to install, works with our theme.... A few little tweaks were needed to which the developer added straight away, support was very quick.
By far the best integrated forum we have seen to date for WHMCS.
We recently purchased this to replace a cms blog we were using, very easy to install and use, works great with out theme etc. Support form vendor has also been very quick.