You can hail a cyclo along major thoroughfares almost any time of the day or night. In HCMC, many of the drivers are former South Vietnamese army soldiers and quite a few know at least basic English, while others are quite fluent. Each driver has a story of war, 're-education', persecution and poverty to tell.
In an effort to control HCMC's rapidly growing traffic problems, there are presently 51 streets on which cyclos are prohibited to ride. As a result, your driver must often take a circuitous route to avoid these trouble spots; the police will not hesitate to fine him. For the same reason, the driver may not be able to drop you off at the exact address you want, but will bring you to the nearest side street. Try to have some sympathy since it is not the driver's fault. Perhaps the authorities would have served the city better by allowing the quiet and atmospheric cyclos carte blanche and forcing the smoke-spewing cars to take an alternative route.
Short hops around the city centre should cost around 5000d and definitely no more than 10,000d; District 1 to central Cholon costs about 20,000d. Overcharging tourists is the norm, so negotiate a price beforehand and have the exact change ready. You can rent a cyclo for around US$1 per hour, a fine idea if you will be doing a lot of touring; most cyclo drivers around the Pham Ngu Lao area can produce a sample tour programme.
You should enjoy cyclos while you can, as the municipal government intends to phase them out.