milk

Is it mandatory to insert a vowel between the l and the k in milk?

I think so. I've heard it [mElk] or [mIlk] but never [ml-k].

<<Is it mandatory to insert a vowel between the l and the k in milk?>>

Yes; the traditional and most common pronunciation is ["mI5k], and many North Americans instead use the alternate pronunciation ["mE5k]; but nobody pronounces it ["m5=k].