Why PBRs were poor torpedo platforms

Could torpedo tubes be bolted to Vietnam-era PBRs?

Not realistically. The boats lacked the structural support for the weight of even small torpedoes and their launch fittings.

The PBR's compact hull was not built to take the structural loads imposed by torpedo tubes and their weapons. Even small torpedoes and their launch fittings would need support that the boat did not have. Bolting tubes to the sides would leave those loads without a suitable path into the hull. The light construction that made the PBR fast and useful in rivers therefore made it a poor torpedo platform.