Using the vacuum pump (for those of us that still have one, who seem to be in the minority nowadays) is an interesting idea.  I'm not sure I would put the extra load on one that was used for the primary attitude information on an IFR plane, but in my case the vacuum system is the backup.  I would think that the pump would produce plenty of air, but I don't have displacement or flow information to back that up.  I think the bleed holes are bigger than .025 as the fuel orifices, as I recall are about that size.  I'm thinking they are more like .030 to .040.  I'd want another check valve to handle the air flow when idling as that is when the air flow is the highest and the vacuum pump is producing the least.  I'd probably set it up to deliver something less than the optimum 3 inches as that would almost double the pressure the pump has to deliver.  No sense in working it too hard, as pilots of P210s found out the hard way.  All I need to come up with is a lightweight, simple relief/check valve and manifold.  And I'm not sure this will work on Continental fuel systems as I don't know what effect back pressure will have on fuel flow calibrations at idle.  The Lycoming systems are mostly insensitive to this.