Vaasa is a summer city so you should absolutely have your visit when the sun is shining and weather's warm. Most likely that would mean visiting from mid-June till late-August.
Hang around the park-like city, spend some time with the locals (who most speak English, Swedish and Finnish and quite a few speak other languages too), enjoy the beaches, check out the islands, go to cafés and so forth.
You might also want to visit Umeå, Sweden. The boatride costs 60 euro one way for adult at this moment (Jan 2010). See the timely prices at rgline.com
