Hi Dave,
I find that the Vixen are an excellent all round eyepiece. I have tested them on different refractors and reflectors and found them to give a very flat and colour free image.
I use them on my 'scope at F11.7 and if anything can show up a bad eyepiece it will be my scope as it is very, very well corrected.
I guess the only way to improve things would be to buy specialist eyepieces for the planets (** Addition here **) otherwise the Vixens are just fine!
Jerry
Addition: Remember, leaving out Earth, there are only 3 planets that warrent expensive planetry (monocentric) eyepieces - for the price of these Registax and a webcam may give better value for money!!