I can't think of many situations where I would use either, either in written or spoken English. I guess I might say "Hi, dear" to my partner when he gets home from work, but it's a rather outdated term of endearment.
I have noticed that 'Hi dear/dears' is a common feature of Indian English, probably derived from 'Dear Sir or Madam', but it is not normally used in most other regional variations.
It should be avoided everywhere. I've noted that it's common on this site (or used to be) among Arabic speakers from north Africa. But it's not correct English usage. "Hi, Dear" - with a capital 'D' because it's a substitute name, like 'Dad' or 'Honey', is only used for intimates. 'Dears' is very uncommon.
You can use it as an adjective, as in "Dear Sir", "Dear Teachers", "my dear friend" (if it is a dear friend).