We have decided to fly to Helsinki on saturday and drive to Vaasa from there. You will probably rent a car anyway in Vaasa, so we thought, to save a bit of cash, why not rent it from Helsinki airport and drive there as its "only" 270 miles. I've just booked the flights for a few of us with BA, although not the cheapest they get there at lunch time which will give us enough time to drive to Vaasa and explore the town in the evening
. I think Ryanair only have one flight a day and this arrives late in the evening (10:30ish) so you'd probably need to stay at the airport and make the journey to Vaasa the next day.