how does one know whether the phrase "viazi vitamu" refers to the sweet potatoes or just delicious ordinary potatoes?

Is there a difference between "nimevipika viazi" and "nimepika viazi"? Are both correct? Does the meaning change? Asante!

Ordinary potatoes in E.A. are sweet potatoes = "viazi". European potatoes are "viazi vya kizungu". So I would translate "viazi vitamu" as delicious sweet potatoes. There are also other varieties of potatoes of which "kirehani" is one.

should it not be translated: I have cooked them sweet potatoes-meaning I have cooked sweet potatoes for them? I have cooked the sweet potatoes could be translated:nimepika viazi vitamu- or am I wrong?