SumOfHIGH = SUM('Vulnerability (2)'[High])

SumOfHIGH2018 = CALCULATE('Vulnerability (2)'[SumOfHIGH],DATESBETWEEN('Vulnerability (2)'[Vulnerability_Discovery_Date],DATE(2018 ,1 ,1),DATE(2018 ,12 ,31)))

SumOfHIGH2019 = CALCULATE('Vulnerability (2)'[SumOfHIGH],DATESBETWEEN('Vulnerability (2)'[Vulnerability_Discovery_Date],DATE(2019 ,1 ,1),DATE(2019 ,12 ,31)))

PctHIGH2018 = ([SumOfHIGH2019] - [SumOfHIGH2018]) / [SumOfHIGH2018]