Vowels can be hard it you slurred them together! But that's okay! There's always a way to oto~ XD Does your P button work? I recommend using that just to find where the second vowel starts and then move preutterance after you find it to where the second vowel begins~
If you recorded them at relatively the same rate, you can probably estimate a value for offsets too, where 1000 is about a second I think? ;v; Also look for where the change of volume seems to be within the two vowels can help~
い is also yo, so did you still have trouble with that since it's not a vowel? D8
Smooth transition might be the UST editing opposed to oto, could you tell me a bit more information?