Castro's exact ailment and condition remain state secrets, but he is largely believed to suffer from diverticular disease, which forms sacs in the colon that can become inflamed and bleed.
In one of a new series of essays he has been writing during his convalescence, Castro said he had undergone numerous operations and the first one had not gone well, explaining why his recovery has been delayed.
Nevertheless, senior Cuban officials have repeatedly said Castro is on the mend and the government has occasionally released photographs and videotapes during his recovery.
State television reported that Castro's meeting with Manh "ratified the warm ties that unite our nations."
Raul Castro and Manh had earlier signed a series of bilateral accords, including one that will make Vietnam Cuba's latest partner in oil exploration and drilling in the Gulf of Mexico.