Fansipan is the highest mountain in Vietnam, and almost everyone now reaches the top by cable car rather than on foot. From Sa Pa town a small funicular runs to the cable car station, and the car itself climbs over the Muong Hoa valley in about fifteen minutes.
At the upper station you are still below the summit. A long flight of steps climbs the last stretch past a cluster of pagodas and statues; a second short funicular exists for anyone who would rather not climb. Allow half a day in total if you want to walk around the temple complex at the top.
Weather decides what you see. The summit is often inside cloud, and a bright morning in town is no guarantee of a view; conditions can change within the hour, so if the sky is clear, go then rather than after lunch. It is also far colder than Sa Pa, close to freezing on winter mornings, and the wind makes it feel worse. Bring a warm layer whatever the forecast says.
Trekking to the top still exists, as a two-day route with a guide and a night in a mountain camp. It is a serious walk on steep, often muddy ground, and it is booked through an agency rather than done alone.