Generally (but, alas, not always) a vowel is given its long sound by adding a silent E.
The short sound of I is the one used in BIT. The long sound is the same as the word EYE.
Ve-hi-cle has no actual or implied silent E in the second syllable (the one with the I in it), so it's short.
Im-pro-vise has no silent E in the first sylable, so it's short. But there is a silent E in the last syllable, so it's a long I (EYE).
Final y in a poly-syllable word is always short (I think!). And it's always long in a single syllable word.
Custody, Happy, but Fly, By, Try, My
Tylenol is a brand name. They have no rules. That's just "the way it is"
If there is a silent e in the sylable (implied or actual, meaning it was once there but got dropped by conjugation with other word elements), then the Y is also long, pronounced as EYE. For example: Eye. Or the British Tyre.
Look at British. Two short I's. Even the shortform: Brit, still short I. If you added a silent e, it would be Brite (bright), with a long I. But then notice Bright itself! No silent E. Damn. There went "The Rule"!
There are plenty of other exceptions. For example, Microphone. It's a long EYE. And when you shorten the word to its common shortform, Mic, you still pronounce it "Mike" (and therefore it's even become an accepted spelling).
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