I have an Istar 127mm f8 achromat and its the best refractor I have ever looked thru , for a doublet and if the R30's are all they claim ( as many people do ) they would be very good indeed .
But yes the Vixen is a nice scope , but I would put my money in the R30 f8 if all out visual correction is your goal as these perform like an f12 instrument .
But then there is the weight issue , from what I have heard the Vixen 140 is quite light but the 127mm Istar is very soild and well made and quite heavy so this brings us to the mount , what are you looking to use ?
Brian.