Like ppebble says, it's trial and error. It all depends on your tastes and the strength of the flavorings you are using. Some of the tobacco flavors I use taste perfect to me at as little as 3%, or about a drop per ml. The vanilla may overpower the tobacco pretty easily, so start with a tobacco mix that satisfies you and begin adding a little vanilla.
I do measure by drops when adding flavoring simply because it's so hard to get the residual flavoring out of the measuring equipment. PG, VG and nicotine get measured exactly, though.
BTW, get an eliquid calculator if you have not already. My favorite is
Scubabatdan's.