"We slept well"
Class 8 (vi-) is often used to form an adverb, a bit like -ly in English. For example:
Anaimba vizuri. = "S/he sings well."
Wanaimba vizuri. = "S/he sings well."
Compare with adjectival uses of -zuri
Yeye ni mwimbaji mzuri. = "S/he's a good singer."
Wao ni waimbaji wazuri. = "They're good singers."