A capital gain/loss is long-term if the stock was owned for more than one year (i.e., one year plus one day). A short-term capital gain/loss relates to a stock owned for one year or less. If your shares vested and were sold on the same day then that would be short-term; however, if the vesting and sale were done in 2017, then you report that transaction on your 2017 tax return.
If you know your cost basis, sale proceeds, date vested, and date sold, then you can just report the transaction as you would a regular stock transaction. What did your SARs offer you? For example, were you given the option to purchase shares at a price lower than the market price? If so, then maybe your SARs are similar to an employee stock purchase plan (ESPP). It all depends on how your company's SAR plan works. If you have more information about your SAR plan, you can follow-up with additional information.
@pdarin
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