Both multipowers and VPPs run the risk of allowing too much versatility, especially if the campaign is largely inhabited by mundanes. I've run FH mages with multipowers before, and it's trivially easy to get the cost of a slot down to 1 or 2 points, giving the caster close to 20 slots and thereby access to half the powers in the list. VPPs, of course, give the caster access to the entire powers list, in theory.
What you're going to have to do is radically limit the versatility, as you have already guessed. Fixing the spells for 24 hours at a time is one way to do that. At minimum I'd require a caster to have to spend at least half an hour to make a change to the pool--anything less than that and casters become unstoppable swiss army knives for noncombat situations.