I had US$ RSUs that vested in 2017 (30 shares minus 10 sold for withholding), I sold the remaining 20 shares in 2017. How do I report and what conversion rate do I use?

I would suggest you reach out to your employer or financial institution that manages your RSU's and see if the withheld amount had already been accounted for in your T4 as tax paid. They should be able to clarify what portion of the stock sold, if any you should be reporting. 

As for 2017 conversion rates, you can find them here https://www.bankofcanada.ca/rates/exchange/annual-average-exchange-rates/