Ahhh I don't know if a city taxi would take you that far -- I guess they would. There are buses from Saigon to Can Tho but they mostly leave in the morning.
Otherwise perhaps a private car with driver could be an option. Perhaps contact some travel agents in Saigon to see what they suggest price wise.
Go to the Western Bus Station (Ben Xe Mien Tay) and you can usually get a minivan there - with the bridge at Can Tho still not complete the wait for a ferry takes about as long as getting there, plan for 5 hours. I recommend teh Mai Linh express buses - a little more expensive but they don't pack as many people in and they run on schedule with little complication.
Since this existing thread will probably keep getting search traffic, I'll ask my new question here...
Do the Mai Linh buses leave from the Minh Tay bus station or from their office in District 10? Also, I keep seeing conflicting information on which bus company might be best for me. I took a Phuong Trang bus from Nha Trang to Dalat, and it was fairly comfortable, but I was the only foreigner on the bus and none of the announcements were in English.
I've quite tall and fairly wide too, so I'm hoping to find the most comfortable bus possible, and I don't mind paying more for it. Hoping to have English-language info on board too, but that's not a huge deal.
Any suggestions on which might be the most comfortable bus service from Saigon to Can Tho, and do they all leave from the same station or not?
I'd say go for the Mai Linh bus.
As for their bus station: in general (if you book through reliable agent) you should be picked up by a Mai Linh taxi to take you to their office. But I guess it varies and I do not know what the reason is for that. Taxi is most times (I never paid and made the trip a few times) free of charge but check that beforehand. And even if you have to pay make sure the meter is on. In general you should be fine.
Practice the words "Can Tho" and you probably won't need more. Only keep an eye on your bus (the number especially) during the short pitstop. You may have difficulty finding your bus among 20 identical green buses.