Given that diameter is equal to height of a cylinder

So, h = 2r

Diameter = 42cm

Height = 21cm

Diameter (d) = 2r

2r = 42

r = 21

Radius = 21cm

Volume of vessel = π(21)^{2} x 21 ..........(2)

Since volumes are equal

Equating (1) and (2)

Radius of cylindrical vessel = 21cm

Diameter of cylindrical vessel = 42cm.