If a vendor became an LLC mid-year, do I need to issue 2 separate 1099-MISC? One under his ssn and the other for his LLC using his FEIN?

[DS30]

Yes - You will need to file 2 separate 1099-MISCs one for the dates that the vendor was a sole proprietorship and one for the dates the vendor was an LLC.

IF the LLC is an SMLLC and treated as a disregarded entity, then you would only have to send one 1099-MISC for the new LLC using either the SSN or the EIN (since the entire amount will be reported on the person's personal income tax return on a Schedule C).

[accounting_datas]

What if it's a single-member LLC (disregarded entity)?

[DS30]

Then you can send just one with the person's SSN or the EIN on it (since it will be included on their Schedule C)