Vantaa is simple to reach from Helsinki. Commuter rail line I (Lentoasema) connects Helsinki Central Station to Tikkurila station in about 30 minutes, with frequent service throughout the day. The same line continues to the airport (Helsinki-Vantaa).
Helsinki-Vantaa Airport itself is in Vantaa, so international arrivals are already in the city — though most will be taking a train straight to Helsinki. Buses and trams connect various parts of Vantaa with the Helsinki metro area, and the city's cycling infrastructure is good.