"Wallah" and "Wallahi" are the same word in Arabic. The latter spells out all the diacritics, but the last diacritic of the last word can be dropped in Arabic so people may drop the "Kasra" (E or Y sound).

When you want to exaggerate a word you usually put extra emphasis on the last diacritic or letter if it's a vowel, so you must sound it out.