Usually all combinations of vowels in English function as diphthongs. Are there any combinations of vowels in English that do not function as diphthongs? if there are no such examples - I would be grateful to you if you mention the source, where this restriction is mentioned as a combinatory rule for English vowel phonemes.
You are absolutely right -my question didn't sound precise. I meant the phonotactics of Old English - were there any cases of vowel combinations within one morpheme that were not diphthongs? Something like CV+VC or CCV+V
(A later comment from the original poster)