Lithuanian diphthongs

Does anyone know under what conditions IE *ei and *oi become respectively <ei> and <ai> in Lithuanian, and when they become <ie>? For example, is it determined by tone or stress? Also, why does <vienas> "one" have <v->? Is it the same reason Russian <vosem'> "eight" has <v->? (Which, I assume, is due to the development of an original [w]-glide before the /o/ in initial position, analogous to the development of /v-/< *[w-] in initial *u: becoming <vy-> as in <vydra> "otter").

Andrew