First up the Ricoh finder is as good and as bright as the Voigtlander viewfinders because it IS a Voigtlander viewfinder... it's an OEM product by Cosina Voigtlander for Ricoh.
Secondly if you are planning on using this viewfinder for both your 21mm and 28mm 35mm film lenses then you need to realise that the brightlines inside the viewfinder are in 4:3 format (which matches the sensor size of the Ricoh GRD) and not 2:3 35mm film format.
Thirdly the finder "foot" on the Ricoh branded finder is offset so that the finder sits to the right of the flash shoe when in place. This is done so that it avoids fouling the Ricoh GRD's pop up flash. But finders designed for use on Leica's generally are offset to the left in order to center them over the lens.
If you were after one finder for the job I think the new Zeiss 25mm/28mm, while definitely more expensive, would do a better and possibly more accurate job.... for the 21mm you could just use the space outside of the 25mm framelines.