Rather than replace rhe Vifa tweeter you might be better of supplementing it with a ribbon used as a semi-super tweeter.I have found this works very well,especially with the 19mm Vifa tweeters.You can just sit the ribbon on top of the speaker and use a 0.5 mF cap to protect it and roll it off below 12000hz[approximately if it is 8 ohm].I would be hesitant to use anything of higher value than 0.5 mF .There is some risk of damage with first order crossover below this point,although I have used 1 mF on my Fountek JP2 without problems.
JT
JT