The milligrams of protein per vial will depend on the units/mg protein defined on the product's certificate of analysis. For example, lot SLCM3898 reports specific activity of 19 units/mg protein. This results in approximately 1.3 mg of protein per vial for this particular lot. The protein content of this lot of material is reported at 61%, therefore the total mass of solid material in the 25UN vial will total around 2.2 mg.
(Link to certificate of analysis: https://www.sigmaaldrich.com/certificates/sapfs/PROD/sap/certificate_pdfs/COFA/Q14/L6007-BULKSLCM3898.pdf)