Deductions & credits

Generally, you can't claim foreign tax you paid in 2018 on your 2019 return. You would need to go back and Amend your 2018 Return to claim the Foreign Tax Credit for tax you paid in 2018. 

However, since you paid tax in 2018 for RSUs vesting in 2019, you can claim the entire amount of the Foreign Tax you paid in 2018 by indicating 'accrued' on the Foreign Tax Entry screen, as long as the tax credit is not greater than the income realized in 2019.

Or, as you mentioned, claim the Foreign Tax Credit in 2019 only for stock vested in 2019, etc. 

Type 'foreign tax credit' in the Search window, and 'Jump to foreign tax credit' to get to the entry area.

Click the link for detailed IRS info on the  Foreign Tax Credit.