Idea-smoothing
Idea-smoothing is a process that occurs in many dialects where bisyllabic /iːə/ becomes smoothed to the diphthong /ɪə/ in certain words, leading to pronunciations like /vɪəkəl/, /θɪətə/ and /aɪdɪə/ for "vehicle", "theatre/theater" and "idea" respectively. This is not restricted to any one variety of English; it happens in both British English and American English as well as other varieties; although it is far more common in the former, as some Americans do not have the phoneme /ɪə/ in their speech. The words which have the /iːə/ to /ɪə/ may vary depending on dialect. Dialects that have idea smoothing usually also have the diphthong /ɪə/ in words like "beer", "deer" and "fear" which are pronounced /bɪə/, /dɪə/ and /fɪə/ in those dialects. Some northern English accents have /ɪː/ instead of /ɪə/ in all words affected by idea smoothing, and some have /ɪː/ for the first vowel in words such as period, serious, etc. but then /iːə/ for the second vowel.
Read more about this topic: Phonological History Of English High Front Vowels