Plenty of tours on this site start in downtown Vancouver or Seattle and put you back there the same night. We list them separately from the island departures, because a coach that picks you up at a Vancouver hotel is a different product from a walking tour that meets at the Empress, even when both say Victoria in the title.
The Vancouver day trip, honestly
The standard product is a coach from downtown Vancouver to Tsawwassen, the ferry to Swartz Bay, a stop at the Butchart Gardens, a couple of hours in Victoria's Inner Harbour, and the same route back. It is twelve to thirteen hours door to door, of which around five are on the coach and the ferry. What you get is the Gardens and a walk around the harbour, which is a fair summary of Victoria's greatest hits and, for a visitor with one spare day in Vancouver, worth doing.
What you do not get is time. There is no room in the day for a whale-watching trip, the Royal BC Museum, Fisherman's Wharf or dinner. Some tours substitute a whale trip for the Gardens; check the itinerary rather than the title.
Faster ways to make the same day
Taking the Harbour Air floatplane or Helijet over and back turns the transit into about 35 minutes each way and lands you in the Inner Harbour, which leaves a full day in Victoria for a whale trip and the Gardens by bus. It costs more than the coach and takes no car, and it is the way we would suggest to anyone whose budget allows it. Alternatively, walk on the ferry from Tsawwassen and take the BC Transit bus from Swartz Bay into town.
From Seattle
The Victoria Clipper leaves Pier 69 in the morning and returns in the evening on most days in summer, with fewer sailings in winter, and gives roughly six hours in Victoria. That is enough for a whale-watching trip, or the Gardens, but not both. Bring a passport; it is an international crossing. Seattle-based coach tours that use the Coho from Port Angeles are longer days with more driving.
When to stay the night instead
If you want to see whales and the Gardens, or you want to eat in Victoria, or you are travelling with young children, book one night. The extra cost of a hotel is often less than the difference between a coach tour and a floatplane, and it removes the last-ferry deadline that shapes every day trip. It also opens up Sooke and the Cowichan Valley the next morning.