More (or, perhaps, 'revamped') ideas:
- Instead of purely investigating corruption, you work for the Council (communicating with them through a represenative of theirs). They also have a stake in the guild war - it's hurting their economy and is bad for their PR. They want it over with as soon as possible.
And thus again we ask:
How do you meet them?
You overhear a conversation in the Council of Six building, with the representitive arguing with someone in disguise (one of the councillors). This introduces the problem, and the representative notices you (in typical BG2 fashion - 'you look like an adventurer, perhaps you could help me'). If you accept, it begins - if you don't, it remains open (until someone else takes it - which would by some coincidence happen at about the same time as you accept either of your other offers

).
What will you do for them?
A few things to weaken the vampires (I believe there is atleast one shadow thief on the council - they would twist this to their own ends as much as anyone else). You would end up fighting Bodhi in the end, for this same reason,
Why do you want to go to spellhold?
Bodhi escaped and the council know this before you come back to them (news travels fast). They believe that she went to Spellhold, where the Cowled Wizard's (and hence, the council's) control has ceased. They want you to go there, find out what's happened, and prevent the guild war from starting up again. The Cowled Wizards can't send you to Brynnlaw unless they control Spellhold, and so you're sent off on a ship - and since money is tight (well, the council says so, anyway), you have to put in $15,000 out of your own pocket, to sail with Saemon Harvarian...