You want the fletchings to be as far back as possible without contacting your face. The closer to the nock gives the fletchings greater leverage on the shaft for more efficient guidance, so if getting the minimum fletching size is important to gain speed or reduce crosswind drift, then move them back. Having the feathers touch your face isn't ideal, it is just contact that can easily be avoided, so if the vane needs to sit forward a bit to achieve this, by all means put it there. Have someone watch you draw a couple arrows fletched at different points and tell you how much clearance you have. Leave a little extra, say 1/8-1/4. Good luck!