If there is no direct bus to A Coruna from Finisterre, and you need to go via Santiago, think about taking the train from Santiago to La Coruna.
Train station in Santiago is easier to reach than the bus station and a lot more comfortable (Santiago bus station is very run down, although I appreciate that if you have taken a bus from Finisterre to Santiago, you are already at Santiago bus station, and you do not then need to walk to the train station). Also, on the train, you have your luggage with you, instead of it being down in a hold of a bus, where you don't know if someone else will remove it on an intermediate stop. Nice new trains too.
Trains go from Santiago to A Coruna at least hourly.
When you come to A Coruna train station, walk straight on across traffic lights from the front door and down a hill to a busy dual carriageway. If you bear right when you hit the dual carriageway, the airport bus stop is about 50 yards further on. This is in contrast to A Coruna bus station which is further away from the airport bus stop (as Bradypus says, the airport bus does not stop at the bus station). Airport bus goes every 30 minutes and if I recall correctly, once I reached the bus stop, I had about 10 inutes to wait.
One thing about the evening Vueling flight from A Coruna to Heathrow. Ryanair type food to buy on the plane. In addition, A Coruna airport is very small - you do not need to get there 2 hours before the plane goes. Nearer to 1 hour before the flight is sufficient. There was a cafe type restaurant by check in, but the food served could hardly be described as wholesome. There was a similar poor cafe through security. Bottom line meaning that as the Vueling flight is a late evening one, I would recommend eating a good lunch in Finisterre or Santiago before you transfer to A Coruna (there are also not good restaurants near the bus or train stations in A Coruna).
Main reasons I flew from a Coruna was that was a direct flight to Heathrow, and the price was much cheaper than flying to Heathrow than from Santiago (via Madrid or somewhere). Santiago though is a good, modern airport.
