High school math class: the hypotenuse is equal to the square root of the sum of the squares of the two opposite sides. Speed being a direct derivative of distance, we can surmise that the same is true for speed also. Therefore:
Speed = sqrt( ( VelocityX^2 ) + ( VelocityY^2 ) )
[ Edit: in honesty I had to Google this, but did so by Googling 'converting two vectors to velocity' - which is as embarrassing for me as it should be for you ]