I've used type of setup you are talking about,(Large diaphram on vox,small diaphram on acoustic).Might I suggest a pair of mics capable of doing figure of 8 patterns.Placed tip to tip,in a Blumlein pair.One lobe of the figure of 8 pointing at the singers mouth,the other lobe pointed at the body of the guitar.
This gives superior off axis rejection,and avoids getting too much spill over, phasing ,and combfiltering.