Are you talking about a full space mirror, or a framed hanging mirror? I don't understand why you need a 8" backsplash? 8" above vanity would be too high and you should be able to cut exception in mirror to accommodate plug and do a mirrored plug plate if you are doing a full mirror. A full mirror should sit right above the 4" backsplash. So, if you have some photos to post that would help understand the space and help us give you better input.

Any hanging mirror should have it's center roughly 54-60" above the floor, depending on height of family members.