"It is not good to sleep in the classroom"
Translation:Si vizuri kulala darasani
'vizuri' can also mean 'well/good'; similarly you could say 'si vyema kulala darasani'. In this context, '-zuri' is an adverb of manner, and I'm like 90% certain that the rule is when you're using an adverb of manner that doubles as an adjective, and thus typically adopts the noun class prefix of its noun ('-zuri', '-ema'), as an adverb it adopts the prefix 'vi-'. I hope that made sense